Hi,
three pointers:
- Check various browsers (Internet Explorer and Edge for example are not supported in the Calendar app)
- Check the WebDAV interface of your browser with the litmus test explained here: How to compile and run litmus (on debian) for WebDAV tests
– > Shared hosters / webhosters are known that they have quite often broken setups for such WebDAV connections which is causing such issues - Check the error console of your browser for possible errors when creating this event