I’ve just added a single calendar into Thunderbird Lightning.
The procedure is quite simple: select if network or local, then insert calendar path, stop.
I was amazed at one fact: after entering the path, the calendar and the tasks synchronized without asking for my username and password.
May it depend on whether the access data was already stored in thunedrbird or what else?
I would recommend to check the account settings in the Thunderbird configuration. If you’h previously entered the data it most likely has been taken from the cache.