sync error -sequence contains more than one element

cpanel is caldav server - onecalendar been working well on android  and windows 10 computers with my cpanel calendar for 6 months+. About 2 weeks ago all onecalendars developed sync error - I think cpanel version may have updated recently to version 120, not sure if this is the issue - caldav and calendar working OK on Thunderbird - no sync error. have checked the caldav url at the cpanel end.

If I remove account from onecalendar and re add this is what happens

and there is now no data in the onecalendar. I cant make sense of the error. Same error on Android app. Please help