Hmmm.. it's obviously not so easy to give the server the right error level, because as I mentioned, I work with a Plesk panel. So therefore I have to adjust the error level for every domain. At the moment the error level is 6143 (whereas E_ALL (excluding the deprecated levels) would be set this way:
php_value error_reporting 6143, see also here: http://forum.parallels.com/showthread.php?t=259634
When I try to set the error reporting level in the Joomla backend to "Maximum" (or whatever <> "Standard"), I got an error message after clicking on "Save": Content-Encoding-Fehler....
I know, it's very strange!