With debug messages, you have to be careful since it pops up a lot of stuff that is not relevant, or at least no error.
In your case, follow the apache-log:
- [04/Mar/2020:11:48:26 +0100] "PROPFIND /remote.php/dav/principals/users/[my_account]/ HTTP/1.1" 401 557 "-" "Thunderbird CalDAV/CardDAV Lightning/68.5.0"
[my_account] [04/Mar/2020:11:48:26 +0100] "PROPFIND /remote.php/dav/principals/users/[my_account]/ HTTP/1.1" 207 776 "-" "Thunderbird CalDAV/CardDAV Lightning/68.5.0"
The first request is denied because access is not permitted, then your Thunderbird uses authentication and it works.