Removed Plugin Statistiken and Blog seems to be free of errors.

After removing timezone Plugin received the next error.
[29-Jul-2025 23:33:42 Europe/Berlin] PHP Fatal error: Uncaught mysqli_sql_exception: Table 'd02a61de.serendipity_visitors_count' doesn't exist in /www/htdocs/w0182e3c/pocosdecaldas.de/serendipity/include/db/mysqli.inc.php:68
Stack trace:[29 ...

Had a typo in the errorlog name. Now got a result. It seems to be the timezone plugin.
29-Jul-2025 23:23:37 Europe/Berlin] PHP Fatal error: Uncaught TypeError: Unsupported operand types: int + string in /www/htdocs/w0182e3c/pocosdecaldas.de/serendipity/plugins/serendipity_plugin_timezones ...

The error logging is exactly what I switched on. I can see it in the PHP Info. I fear the error is not shown because a 500 is just a HTML error. At all-inkl I have to wait for the next day to get the html access logs. I will come back tomorrow.

Sorry for the mismatch. You are right with the 500. My hoster is all-inkl and I switched on the PHP errorlog for the serendipity domain. Meanwhile I tried all available PHP versions (8.1, 8.2, 8.3, 8.4) all with the same result 500.
At the moment my dashboard bottom line looks like this:
Betrieben ...

I upgraded PHP 7.4 to PHP 8.4. Ater this I can still open the Admin Desktop Page but not the Blog Page. Error is 500 page not found.
Going back to PHP 7.4 let it work again. I did not find any errorlog.

Any ideas?

Trying to add an object from the media gallery is not working under the Chrome Browser. When I am pressing the "ready" button in the the editors add media popup nothing happens.
Looking into the Chrome Debug view I can see the following error:

Uncaught DOMException: Blocked a frame with origin ...