While configuring my CalDAV accounts in Thunderbird on my new laptop, I ran into an issue. I have a few CalDAV accounts that are all on the same Zimbra server. Thunderbird only requested a login and password for the first account I added. It tried to open the next account I added with the credentials that were already stored for the first account, and that failed.
Since this worked just fine on my workstation, I suspect the default value for some setting was changed in a newer version of Lightning. That, or I changed the setting on my desktop as well and didn't remember it :-) Either way, I am documenting it here now.
The fix is simple: go to Edit > Preferences > Advanced > Config Editor. Click the "I'll be careful, I promise!" warning, find "calendar.network.multirealm", double click it to toggle it to "true" and finally restart Thunderbird. You should get an auth dialog for all configured CalDAV accounts.