If you can create the account and the calendars are listed. Then, the issue is coming from a failing sync attempt.
For CalDAV accounts, you may gather logs from failed sync attempts following instructions in this wiki page:
https://sailfishos.org/wiki/CalDAV_and_CardDAV_Community_Contributions
Then, you can copy-paste here relevant parts of the log, particularly the parts where the error is mentioned.
In case you would like to nuke the calendar database, close the calendar app and remove or rename the following file ~/.local/share/system/privileged/Calendar/mkcal/db. I’m not sure that it will help, I more inclined to think that the issue is coming from the CalDAV sync, with an event that cannot be imported, blocking the full process. See this thread about about possiblity in the future to better handle these cases : Calendar sync should continue with next item on error