@thunderbird A hard-earned lesson about #Thunderbird 115:
before upgrading to it, disable or perhaps even completely uninstall the Cardbook Add-on, if you had it installed in your previous version. It will conflict with the new DAV powers which Tbird 115 has built into it's Addressbook.
It (Cardbook's continued interference) caused serious account/password problems for me, for an entire Nextcloud account I had been using as a #CardDAV Addressbook source. Conflicting overlapping logins, it seems.