I run into the same error than @ oatmeal:
OCA\Mail\Exception\MailboxDoesNotSupportModSequencesException: Mailbox does not support mod-sequences
Mail Version 1.8.1, Nextcloud Verision 20.0.7
The symptoms: Mails are loaded, some seconds later mails disappears, some seconds later they appear again - endless loop.
German native, English so-la-la, Italian what?
This seems to be expected as a workaround - but either it is not merged into 1.8.1 or it is not working for all cases of this exception
I checked the patch above - it is in 1.8.1 and it seems it doesnt work
Story continues on github: