I use the em-client with my Mailbox.org account. I also sync our Zoho family calendar. The em-client provides the following error. “Subfolder synchronization for folder user failed due to the following error:” with nothin after the colon. I attached screen shots of the error and the sync protocol. I can also send you the log file.
Yours, Krimhild
I think the remainder of the error is “not implemented”. Check in Menu > Tools > Operations, then in the Log tab. You should find the full error.
It could be that sub-folders are not supported.