Charles G. Herbermann, "More Light on the Election of Urban VI,"  American Catholic Quarterly Review 18 (1893), 407-411:

(Littera Gebennensis ad imperatorem Karolum de commendatione et creatione domini Urbani pape sexti):

Serenissime princeps et domine consinguinee Karissime.

Post felicis recordationis occasum domini nostri Gregorii pape XI., quem per alias litteras meas serenitati vestre lacrimabiliter nunciavi, aliis dominis meis cardinalibus et me hic existentibus, revolutis X. diebus post obitum secundum iuris canonici ritum, clausis in conclavi in archiepiscopum Barensem tunc, nunc summum pontificem natione Neapolitanum, qui domini mei cardinalis Pampilonensis vice-cancellarii apostolici absentis vices gerebat in curia dicti domini mei et cetera unanimiter direximus voces nostras eundem ad apostolatum eligentes VIII. die mensis huius in conclavi solummodo unius noctis mora protracta, quod Romani in longioris temporis in dicta conclavi moram consentire noluerunt. Vocatus est Urbanus sextus m[agister] dum erat in minoribus, domesticus et apostolicus quamvis de gradu infimo nunc sublimatus ad supremum, cuius coronacio in paschate resurrexcionis domini proxime venture est ordinata celebrari. Ipse enim de serenitate vestra multum sperat et quod sic aliis predecessoribus suis eadem serenitas filia fuerit et iuvaminis prachium singulare sic in eum constanter debeat maiestas vestra. Et quonium [sic!] tum et in factis tangentibus serenitatem vestram et serenissimum natum vestrum super quibus cum eo strictissime pluries sum locutus ipsum reperio dispositum valide bene adeo quod si opera verbis confirmaverit, sicut spero, negocium dicti serenissimi nati viri feliciter expediet. Ad cuius expedicionem toto conamine meo ipsum sollicitare non desistam, in quibus et Magister Conradus serenitatis vestre secretarius cum omni sollertia commendabiliter laborat, recommendans me semper serenitate vestre quam conservet Omnipotens feliciter et votive.

Scriptum Rome die XIII. Aprilis.

 

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The author observes that the name Ruperti is inserted after the word littera "by a later hand in other ink". Presumably, the heading and the text of the letter are in the same ink, demonstrating that they are not an original document, but a copy of a document.

The letter describes the Conclave as follows, "unanimiter direximus voces nostras eundem ad apostolatum eligentes VIII. die mensis huius in conclavi solummodo unius noctis mora protracta, quod Romani in longioris temporis in dicta conclavi moram consentire noluerunt." Hebermann remarks, "This letter, the reader will observe, while adverting to the pressure brought upon the Cardinals to shorten the conclave, gives not even the slightest hint of any violence curtailing the liberty of the participants." In truth, the letter goes out of its way to remark that the Cardinals did not continue the business of the Conclave beyond one night, because the Romans did not want the Conclave to go on any longer than one day. That is very strong pressure indeed. Moreover, the author is trying to prove the negative, that there was no pressure. Only by a suppressio veri, omitting a good deal of evidence from many reliable sources on both sides, can he make his unsupportable claim. The statement in the letter that the vote was unanimous (a translation accepted by Hebermann on p. 410) is proof that Robert of Geneva was writing under pressure. Two cardinals at least did not vote for Bartolomeo Prignano, Cardinal Orsini (who refused to vote at all, due to timor and impressio), and Cardinal Corsini (who had begun the morning's voting by nominating Cardinal Tebaldeschi, the Cardinal of S. Peter's, a Roman).  In addition, Cardinal Bertrand Lagier (Atgerius), on April 6, during the Novendiales, before the Conclave had even begun, swore out a notarized and witnessed Protestatio (Baluzius II, 815-821), stating that his judgment was that a pope should be chosen from among the Cardinals, not from outside, and that he was decided on naming three Ultramontane cardinals, and that if it turned out that he should nominate any Italian or anyone outside the Sacred College, it should be understood that his vote was being cast out of fear of death and did not represent his proper intent.

Herbermann wonders whether the document is authentic. "Its preservation in the Vatican Library," he says, "is in itself an argument in favor of its authority." The author has a doctorate in law (LLD). He cannot be that naive. On his 'reasoning', the Donations of Constantine are authentic. He is clearly a partisan of Urban VI, and writing whatever will vindicate the Urbanite cause. In fairness, he attributes the idea to Ludwig von Pastor (p. 410), but that convicts Hebermann of uncritical reliance on authority.

Hebermann speaks of a reference in the letter to Conrad of Wesel, the Imperial Ambassador. Conrad Henrici, Dean of Vysehrad, was the Imperial Ambassador to Pope Gregory XI, and he did indeed inform the Emperor that Prignano had ordered Robert of Geneva to write about the election after the Coronation:

Post hoc certa die DD. cardinales fuerunt constituti coram dicto B,. et ego cum sociis meis, qui eramus ex parte imperatoris pro facto filii sui, videlicet, pro approbatione romanorum regis. Tunc idem B. dixit alta voce, "Domine Gebennensis [Robert of Geneva], ego volo quod vos scribatis D. imperatori consanguineo vestro et regi Bohemiae ac aliis consanguineis vestris. Et vos etiam, D. Florentine [Pietro Corsini], vos etiam promovens facta D. imperatoris, et vos alii DD. cardinales similiter (quorum multi erant ibi) scribatis D. imperatori et aliis amicis vestris de mea electione et coronationis canonica, et ego scribam similiter. Et videatis quod concordetis mecum, et antequam sigilletis litteras vestras, volo eas videre, si concordant cum litteris meis. Ego expediam istos imperiales et mittam nuntios meos cum eis, et mittam D. imperatori et regi Bohemiae raubas meas et etiam aliis principibus. Et non tardetis cum dictis litteris."

Note that it is to Robert of Geneva that the command is given first, because of his royal relatives. Clearly, the letter that Hebermann is discussing as written after the Coronation (on April 19) is not the letter that Robert of Geneva wrote on April 13. There is nothing, however, to forbid Prignano from extracting two letters from Robert of Geneva, one on April 13 and one on April 19 when all the other cardinals were being pressured to write. Rather than considering that possibility, Hebermann chooses to attack the veracity of Conrad of Vysehrad, and (as usual with the Urbanists, rejecting his testimony on the grounds that he was prejudiced in favor of Robert of Geneva). Moreover, Hebermann seems unaware that Robert of Geneva had fled from Rome on the evening of April 8, the day of the alleged election. He was securely lodged in the Castle at Zagarolo, and did not return to Rome until April 11 or April 12. Zagarolo was the property of Bishop Agapito Colonna, who was sent by Prignano-Urban to get Robert of Geneva back to Rome:

Eodem die [Saturday, April 10, 1378], D. Agapitus [Colonna] redit de Zagarolo, ubi D. Gebennensis [Cardinal Robert of Geneva] erat, et retulit D. N. quomodo D. Gebennensis promiserat omnino nocte sequenti se venturum [April 10/11], et quomodo dixerat sibi: quod iste erat verus papa, et jactabat quoniam ipse fuit causa creationis suae. Et illa nocte D. Agapitus remansit in palatio

Cardinal Robert of Geneva had not participated in the enthronement of Prignano on April 9, nor in any of the 'adorations' or 'recognitions' at all. It was essential for Prignano to get Robert's acceptance of him as Pope on paper with his signature and seal as soon as Robert returned to Rome. Hence, the desire to have a letter written to the Emperor—which took place on April 13.

Robert of Geneva later spoke to Ambassador Conrad Henrici and asked him to write to the Emperor, to the effect that he should not believe Robert's writing on the subject nor those of other cardinals:

"Conrade, tu vidisti et audivisti omnia que facta sunt ante conclave, in conclave, et post, et qualiter romani inhumaniter tractaverunt nos, et compulserunt nos ad dandum eis romanum vel ytalicum, et quomodo nunc etiam iste compellit DD. cardinales ad scribendum suam electionem fore canonicam et coronationem D. imperatori et regi et aliis principibus et notis nostris contra veritatem et conscientias nostras et contra omnia jura, et, si non faceremus in hoc voluntatem suam, sine dubio essemus in periculo mortis. Igitur rogo quod velis te parare ad viam eundi ad D. imperatorum ad informandum eum de veritate et de omnibus predictis violentiis seu impressionibus, molestiis et contumeliis illatis et factis Dominis Cardinalibus, quia ipse credit tibi, et quod non advertat nec curet scripturam meam circa hoc, neque aliorum cardinalium, quae sibi scribuntur, nec etiam credat nuntiis ipsius B., et dicas sibi audacter et secure quod cito fuerimus extra Urbem in libertate constituti, procedemus ad electionem canonicam futuri romani pontificis. Et deinde expediamus factum filii sui et scribemus cum nuntiis nostris solempnibus totum factum et veritatem quod nunc non audemus facere, quamdiu sumus in Roma."

Hebermann seems unaware that there were two Conrads. One was Conrad Henrici, Dean of the Chapter of Vysehrad, Imperial Ambassador. The other was his colleague Conrad de Gysenheim, who was procurator and Secretary of Wenceslaus, the King-elect of the Romans and King of Bohemia. Hebermann's analysis of the letter and its significance cannot stand against the known facts.