OK, I got it to work. The language setting is not used to determine location (e. g. Austria is European Union, but German is the language), and shipping entries are one of four languages.
Instead building on what you mentioned before, I unset the germanShipping session and enabled euroShipping. It works now. What baffles me is, that it has worked for many years without the change.
$_SESSION['euroShipping'] = on;
Thank you :)