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_close are required if you want Craft 3 to automatically backup your database because it uses the native
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.