Can't move/delete folder "Renaming not supported across conflicting directory permissions"

Dear all,

several users have reported that they were unable to move/delete folders using Thunderbird and they got the message "Renaming not supported across conflicting directory permissions". When I google for this message, I see hints like "your mail may not be all on the same volume", but here this is definitely not the case. All mail is in /var/mail/virtual and owned by mail:mail

Curiously, when I tried the same operation in Roundcube, there was no error reported, so something about the combination of Thunderbird and Dovecot seems to be causing the error.

Does anyone have advise on how I should go about tracking down the cause of the error?

Thanks a lot in advance!

Johannes