iMac Calendars app error with NC12

Hi Folks,

I’m running Nextcloud 12.0.3 with Calendar 1.5.7 app.

I have added it as a CalDav account in to the Calendar app on my iMac (High Sierra)

I imported a load (4k) events from my google calendar and they seem to have moved over to Nextcloud - but it was throwing the attached error.
Clicking Ignore and Try Again only closed the window (and clicking the little traingular exclamation mark error icon brought it back up) Clicking Revert to Server seemed to move it on to the next one.

Any clues what the problem is or how to solve it?