My Calendar Plugin Problem in WP Multisite not always linking to correct events pages

Hi all
we have a strange problem with a WordPress Multisite and MyCalendar

https://wordpress.org/plugins/my-calendar/

On some events (not all) that calendar is not linking to the correct events page and instead shows an error page 404 or links to a wrong page, even we create that event new. The same event linked from another subsite links correctly without any problems.

What we have done so far.

Checked that all subsites are set up with my calendar widget the same way.
No differences here anymore!

We checked all settings of my calendar plugin on all sub - and main pages.
No difference here anymore either!

We updated the complete WordPress installation and all plugins to its newest versions.
Did not help!

We deleted all transients and cleaned up the tables from former parts which are no more available in the installation right now. - we took over the site from somebody else!
Did not help either!

We would like to know if somebody can name some other ideas where we could check why an event we create on the main site installation gets displayed correctly on some sites and does not link to other sites back from the event entry to the actual events page. What else could cause that linking problem in a Wordpress Multisite?

We will now start searching directly in the database for a hint why that isn't working like it should. For me, it looks like that old data gets read even an event gets created new, but from where is WordPress pulling that wrong data which is no more valid. It is like a WordPress Hid and Seek game grrrr...

Besides transients and Option Table, the setup of the Pages, the Widget, and Plugin configuration where else should I look at?

I like to take some more notes as I am sure that we can solve the problem but I simply don't know right now how :wink:

Thanks
Andi