Thank you for the report.However this is a bug in OC whch has to do with asomehow weird format of saving image data. Please follow up there:
Problem with sync between android and web app
I installed nextcloud 11.0.2 on a mutualized hosting ovh. The cloud works correctly. I have a problem with the synchronization between my android smartphone and the Web application.
I installed davdroid from f-droid and I configured the .htaccess with rerouting .well-known. I obtain my address books and my schedules but no synchronization. You can help me please.
503 Service not available seems to be a server problem. Did you ask Nextcloud support?
But synchronisation work between thunderbird (lightning) and (two direction) web application.
@TiCaillou Then it seems like the server error only appears in responses to DAVdroid requests. Did you ask Nextcloud support?
It working with caldav sync adapter but i have not addressbook.