Ok, I have another hint now. After I set the log level of apache to info, I get the following message:
[Thu Apr 13 12:53:39.298567 2023] [http:info] [pid 584232] [client ---ip---] AH01588: Requested content-length of 1145550174 is larger than the configured limit of 1073741824, referer: ---url---
That leads me to this parameter: LimitRequestBody
The default used to be 0 = unlimited but is now 1GB. I added LimitRequestBody 0 in /etc/apache2/apache2.conf and it
works now.
I am very happy that this problem is now fixed. And appreciate the help and hints I got