Thanks for the tip. The workaround doesn’t work for me (first deleting the saved password, then changing the “network.cookie.same-site.enabled” value), however.
Note: I try the DAV URLs of:
https://myserver.com/remote.php/dav/addressbooks/users/myuser/contacts/
…offered by my Nextcloud contacts Settings, as well as:
https://myserver.com/remote.php/dav/addressbooks/users/myuser/Thunderbird/
…which I hand-crafted myself, as there is no “impeller” icon to be seen, as suggested in the Documentation here (look for the red square there).
No luck trying either DAV URL, I get the same “Validation” error messages as before. I’m glad to know the CardDAV developer as well as Thunderbird themselves are well aware of the problem. I think if I just wait long enough, the fix will come down the pipe with a Thunderbird upgrade (I’m in Linux Mint 19.1).