This Solution does not work if there is a mod_security installation problem like this:
Installation started in background
Auflösung der Paketabhängigkeiten wird geprüft.
Pakete werden installiert
Reading package lists...
Building dependency tree...
Reading state information...
Check entries in /etc/sw-cp-server/config - http-Block:
http {
fastcgi_buffers 32 32k;
fastcgi_buffer_size 64k;
fastcgi_read_timeout 600;
fastcgi_send_timeout 600;
}
if not like this - Edit and save.
Before i had 16 16k and buffer size 32k
After that change, never had problems again.