gpg: decryption failed: Bad session key

I found this, which seems to be the magic that I could not find anywhere else:

Try adding --pinentry-mode loopback to your command. Maybe there is a problem with your installation / pinentry program so that it does not start or you are accidentally using a dummy / test pinentry which provides the wrong passphrase.

For the record, if somebody will encounter this problem, too:

The problem was, that the encryption was done using gpg version 1.4.11 and the decryption was using gpg version 2.0.22.

After upgrading the encryption to gpg2 (2.0.17), everything worked fine.