Using CM 3.17.8 on Ubuntu Linux (flatpack), together with a Zimbra Server 8.8.15. Configuring a new email account from scratch, CM connects to the IMAP server and scans the folder tree. This operation does not complete at all. From the log I can see a log of LSUB / OK LSUB on the same folders, ever and ever again. Left CM work for hours, no change. The same account works perfectly on Thunderbird and other IMAP MUAs. Apparently the recursion is stuck on a 5-levels subtree. Tried re-creating the account many times, no change.
Not sure it's related, but after killing the folder tree (re)build, I've switched off the "Subscriptions > Show only subscribed folders" and the process started again but stopped after a while...