Issues with the Calendar view

We understand your problem.
But first it is important to know where the error might be.
By first choosing the default template to display the page, you can exclude whether it is in the programming of the page or in the programming of the template.
There are many examples where templates are not completely built according to Joomla rules, which causes certain problems.
Sometimes it can be solved by inserting a line of text somewhere, but first it has to be clear where...
Hence the request: try the default Joomla template first and then report the result...

Thanks I setup the site again and updated the Joomla 3.10.1 to test the issue.

But I could not install the JEM on Joomla 3.10.1. When I will be able to install the extension than I will check with the default template.. I created a new topic for this ( www.joomlaeventmanager.net/forum/jem-2-3-x-for-joomla-3-community-support/11548-installed-failed ).