Hello,
I struggled with the same error for quite some time (longer than I care to admit actually), and some how lucked into the solution while chasing down another issue… I’m not sure of your web config, but if your NC root dir is pointed at a subfolder ( i.e. /var/www/nextcloud/[your_nc_sub_dir]) be sure the .well-known redirect follows the same dir structure.
You didn’t tell us what you changed since then? Update of Nextcloud or OS X? If it is a OS X update, it’s possible that they now check for these .well-known paths…
You can also check your logfiles what happens on your server. The .well-known-stuff would be visible in your access.log of your webserver, your OS X caldav-client is asking for it and gets a error (404, 403). You will also see, if there is any activity at all, in case you have a problem in your home network (probably a problem of foomep).