Lately, I was trying to connect to two CalDAV shared calenders, hosted on different instances of ownCloud, accessible with different credentials.
On startup, Thunderbird was opening several login masks, asking me to enter the credentials for each of the calendars. Unfortunately, I wasn’t even able to distinguish which credentials I had to enter, because the TLD of both calendars was the same, and for both calendars, Thunderbird just showed me the same realm, namely “ownCloud”.
To avoid such confusion of login credentials, and the necessity of re-entering the credentials every startup of Thunderbird, just because it mixes up the realms because of similar realm names, you have to set the calendar.network.multirealm option of Thunderbird to true. (How to enter the configuration editor is described here: https://support.mozilla.org/en-US/kb/config-editor).
After a restart of Thunderbird, it asks one last time for the credentials of each account, but now it distinguishes the logins.