Thunderbird calendar on cloud webdav server only read only?

I am running Thunderbird v91.10.0 on KDE desktop and working on my calendars. I use google calendars for sharing amongst colleagues but wanted to have a shared private calendar. My cloud service (pCloud) provides a webdav server and I have started to try and use this.

I am able to create a new calendar on the cloud server but when I try and use it I am unable to enter an event. If I try and save an event I get an error message and when I then look at the calendar properties I find that the settings have changed to Read Only.

I have opened port 443 on my firewall but now I am stuck so asking for some help please.
Where am I going wrong?