reported as Bug 38475
zdesktop hangs with
Failure communicating with remote server. Please try again later.
if there's a one-off problem on the server end, rather than ignoring the unsyncable item and syncing everything else.
Basically zdesktop is trying to download something that it shouldn't - a file that's not in the LSUB list, and isn't a mailbox, but is in the user directory on the server
The IMAP server logs "Fatal error - header size inconsistent"
I can't see the actual IMAP sequence because zdesktop doesn't support "no TLS".
Obvious workarounds are to remove the problem file from the server, or to reconfigure the mailstore so that it contains only mailboxes. But other IMAP clients such as Thunderbird, Pine do not have a problem