The observations were made with 2 hosts running 4.13.1, connected to an IMAP server running dovecot. The server was a Courier one months ago when the computers were set up, but was converted to Dovecot and no problems have been seen so far. The hosts used different KDE SC versions over time, but were regularly updated to the most recent stable versions.

Reproducible: Didn't try

Steps to Reproduce:
On host1 I opened the addressbook, and edited some contacts in a subfolder. Only this subfolder was selected in the folder list, so only it's contents were showing up. I added some birthdays and other information. Most contacts had images attached to them. I then clicked "synchronize folder" from the folders context menu. Result: all contacts lost images and birthdays.

I then booted the other host. Most contacts were showing up twice, none had an image (they should have had them, the images were added months ago). I clicked "synchronize folder" there, it resulted in the duplicates disappearing and the images showing up. But no birthday entries were seen anywhere. I then added a birthday entry to one entry again on host2, clicking "synchronize folder". The birthday entry was deleted again. Syncing on host1 did not result in any change.

Final state so far:
-host1 has neither images nor birthdays
-host2 has images, but no birthdays
-server has state of host2
-adding birthdays only has an effect until folder is synchronized