The original poster solved this in a follow-up comment:
After checking the log file - see my other question -, i activated the php functions "proc_open" and "proc_close" at my hoster's php settings and it worked. I'm not sure if i can leave these settings.
proc_open
and proc_close
are required if you want Craft 3 to automatically backup your database because it uses the native mysqldump
and pg_dump
binaries to perform the backup.
If, for whatever reason, you can't enable those methods, your options are to:
Set the backupCommand config setting to false
. This will prevent Craft from performing database backups and you'll be responsible for manually creating them.
Set the backCommand config setting to some other script or service that can perform a backup without using those disabled methods. Craft will respect whatever is set in this config setting.