Make sure that config.php and lang/English/common.php exist and don't trigger parse errors (I think display_errors is disabled on your server as well, so I couldn't check that myself).
Thanks for the quick feedback!
I checked for lang/English/common.php . It exists. There seems to be code that sets
$locale = 'en_US.US-ASCII'
since the server OS is FreeBSD.
So, it seems to be the character encoding. Originally, I tried to use the config.php which was generated by install.php . Then I tried downloading it and re-uploading with BBEdit using different encodings (UTF-8, ISO-Latin-1). When I tried to redownload using Western(ASCII) encoding, I got errors telling me that there were unmappable characters.
Now, I set BBEdit to use Western(ASCII) as default encoding, rewrote config.php by hand and uploaded. And now it works!
Still a bit puzzling why the originally generated config.php didn't work...? Oh well...