Thunderbird issues with read status

[Stefan Froehlich]

Thunderbird client seems to not updating the read status properly. When 
an email is marked as read or unread it doesn't sync through to 
roundcube webmail or active sync clients nor to another thunderbird 
instance on another computer. Pressing F5 in Thunderbird reverts the change.

Marking an email in Roundcube or on an active sync device works fine and 
the changes are synced to the Thunderbird clients as well.

This was working before and started only recently but I cannot tell 
after which update it started.

Debian 8, Kolab 16

Any idea where to look at?