I don't think a calendar can be defined as default for manually entering new events; a new event is added to the calendar that is selected in the calendar tab. But there may be an issue with TB not remembering which calendar is selected. If the selected calendar is not remembered between TB restarts, I would delete the session.json file from the profile folder, with TB closed, and see if that changes anything. Help/Troubleshooting, Profile Folder, Open Folder, to open the profile. The session file stores the layout and open tabs that were in place at last TB closing.

https://groups.google.com/g/mozilla.support.thunderbird/c/ztx9VapJxO8

It might also be caused by how the network calendar is added to TB, with the Provider (Google) or TbSync add-ons, or directly as a CalDAV calendar.

Closed Thunderbird, removed session.json, restarted Thunderbird, and reopened the calendar. The local calendar was highlighted in the calendar list pane. I highlighted the network calendar, closed Thunderbird and restarted it. The local calendar was again highlighted instead of the network calendar. Tried this a few times with the same result each time.

Note, the network calendar is Google via Provider.

Also, this used to work, i.e., the network calendar was always selected by default when starting Thunderbird.

I'm not sure in which version this problem began occurring.

Also note, the system on which it doesn't work is Windows 10 Pro. I have a duplicate account on a Windows 8.1 Pro system, and it does work there, i.e., the network calendar is always selected by default when starting Thunderbird.

And, I tried copying the whole profile folder from the Win10 system to the Win8 system, and oddly there is no problem on the Win8 system...

Try it in a new profile with the calendar added directly as CalDAV, no add-ons:

https://support.mozilla.org/en-US/questions/1304356#answer-1368791

If the selected calendar is maintained, you can remove the Provider add-on in the other profile and and the calendar the same way. Help/Troubleshooting, about:profiles, to create and manage profiles.

I was able to create a CalDAV calendar, but could not enable it -- not sure why...

I have a Google calendar added with CalDAV and it can be enabled from the context menu, Properties (see picture). Maybe you have to assign a mail account.