Numerous "unnamed" calendars

In my Calendar which I sync with a number of Apple products (3 iPhones, 2 iPads, 2 MacBook Air, 1 MacBook, 1 Mac mini) I see numerous calendars show up that are titled “unnamed”. I have tried deleting them on one iPhone, on one MacBook Air, the Mac mini and in owncloud itself, but still they do show up again after a few minutes. I have no clue what is going on here.

Since I still run a very old version of own cloud (7.2) which I failed to upgrade, I am thinking the best might be to completely do a fresh install. The only thing holding me back is the thought of how to migrate the data.

Any suggestions?