Hi there!
I did a search for this on the internet, here on the forum and on the github page of the calendar extension but found nothing about that issue I have.
I newly installed SolusOS and Thunderbird and added two CalDav calendars from my NC 11.0.3 account. One is named “Privat” and works but the second is named “Geschäftlich” and does not work. I can add both calendars but second one just doesn’t show any entries although in the upper left mini/month calendar some days are in bold typo showing that there are entries.
I have no problems on Thunderbird on Windows 10 and also no on Android with DavDroid.
I guess it has something to do with the german umlaut “ä” that is simply not shown in the calendar link.