I’m running the latest Fedora 35, php 8.0.17-1, httpd 2.4.53. Seemingly almost spontaneously late last week, calendar stopped syncing, and when I went into the Admin Overview, I got the helpful “webdav interface appears to be broken” message. No real help in the httpd error logs, nor in php-fpm www log. I completely reinstalling Fedora 35 and nextcloud from scratch on another VM, and did not get the errors. Was about to post logs and such here, but in a “hail mary”, I swapped my php.ini around and discovered that it was the line enabling zlib crompression (zlib.output_compression = On) that was breaking everything. I can only assume that something broke in the PHP update.
Anyway, posting here in the chance that it helps someone else with a similar issue.