[bug] caldav sync stops working every now and then

I have a strange Problem here: I sync my Jolla with two different caldav accounts. One is an owncloud account, the other one is an egroupware calendar. For a long time all went fine. But since January syncing the egroupware calender stops working from time to time. The only way to get it working again, is to delete the account from my Jolla and set it up from scratch (Deleting the account the last time, left the calendar undeleted in the calendar-app - But that's another problem, and I think there is already a bug-report).That happened now the 4th time to me and it is really annoying, because I can't rely on that calendar anymore. I know there are still a lot of threads which are referring to caldav, but I haven't found one which describes a problem like mine.