Aerc has problems with non-ascii directory names over imap.
Namely, a directory containing 'ü' at one provider works just fine.
A directory for another provider called
Odeslané fails with aerc saying:
Mailbox delimiter must be a string
Offlineimap converts this name into
Not sure how to provide more debugging info.
I think the problem is not with non-ascii names but rather in go-imap: https://github.com/emersion/go-imap/issues/329
("Odeslané" sounds like something from seznam.cz)