Because I got a wall of silence/lack of response here, I posted question 1367925 at the Mozilla forum, where people are more helpful at least.
The solution was quite simple after all. You simply have to add Google calendars without URL (empty location-field) in the new versions of Thunderbird this is the case since version 78.7.1 if I'm correct.
Because of this new behavior, the previous correct URLs apparently became unusable, obsolete or something alike. Thus leading to the trouble I had connecting to the contacts birthday-calendar using Thunderbirds build in CalDAV.
You can argue this is no bug but a human error. Well, on the other side I used these URLs for years without any problem. So who changed this behavior? Considering the fact it occurred after a program-update, the simple solution in new program behavior and the non-communication here, I think I know the answer. But I leave it up to you to decide.
Thanks for non-supporting me, my issue is solved. This bug can be closed!