Cannot delete folder

[Yves Goergen]

Hi there,

I cannot delete IMAP folders in new mailboxes. When trying to delete a 
folder in Thunderbird I get the error message "renaming not supported 
across conflicting directory permissions". Looking up the error on the 
web led me to checking the mailbox directory's permissions. They're 
inconsistent in new mailboxes. While every single directory in an old 
mailbox has "drwx--S---", some of the directories in a new mailbox have 
"drwxr-sr-x" instead. I'm not sure what that means. The folders with the 
later permissions are "Drafts", "Sent" and "Trash". Folders with the 
first permissions are "Junk", "Archives" and "Archives.2018". All 
folders were created by Thunderbird automatically. Now I wanted to 
delete "Archives.2018".

What's the issue here? Why do folders created by the same MUA in the 
same event on the same mail server have different permissions and why 
can't I delete one of them (which would probably move it into the trash 
folder first)?

Dovecot version is 2.2.22 on Ubuntu 16.04, Thunderbird is 52.6.0 on 
Windows 10.

-Yves