To-Do creation generates error

Hello Community,

I have a problem in related to the to-do management.
I am below the menu item “To-Dos”:

When I select the “New” button and enter my data and then press “Save” or “Save & Close” the following error page appears:

Not Found
The requested URL was not found on this server.

Apache/2.4.46 (Win64)
PHP/7.4.14 Server

But if I add a task via the button “Add To-Dos” which is directly above the existing to-dos (not via the “New”-Button) then the task will be saved. I can then also edit it via the 3 Dots and save it successfully.
Only the new creation via the “New”-Button does not work.

This seems to affect only To-Dos, with the other menu items I have not noticed.
The problem seems to exist since the installation.
Initially I had V2.1.5 installed. But have updated to V2.1.7
What can I check?

Thanks in advance!

Any errors in resources/logs/?

No, not really the “error.log” is empty.

Just a little hint to the fix

would have helped.
I have made the adjustment. Works now.