Thank you for your help, 2112.
However, if I am honest I do only have a faint idea of how to accomplish what you describe. Does the bitcoin client use different encodings on different platforms? The wallet was encriptded on a German Windows 7 installation; for running Revalins script I now use a German Ubuntu 12.04.
Do you have a concrete strategy how I could get Revalins script running using all characters on my keyboard?
Thank you
Well, you had a good idea to see if you can crack the known short password with umlauts.
1) I currently don't have access to any other machine except my single laptop, I really can't help you with details. In particular I'm almost illiterate in German.
2) verify if the "Language for non-Unicode programs" in Control Panel is still "German (Germany)".
3) using regedit.exe verify the settings in HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Nls\CodePage\ACP. It should probably be 1252 which means the Windows-1252 encoding was used for non-internationalized programs like Satoshi's client. Also note the OEMCP value from the same page. Ostensibly OEMCP is used only for DOS compatibility mode, but some programs use it because of bugs.
4) If you encrypted your wallet using bitcoin-Qt then verify that you can decrypt it from the command line by using bitcoind.
5) Make sure that the step 4) works for
all umlauts you may have used, both lower-case and upper-case.
6) configure Ubuntu's terminal program to use Windows-1252 or if you access it from Windows via ssh configure your ssh client to use that encoding
7) rerun the test decryption of the known-password wallet on Ubuntu's command line.

verify that your Ruby program is using the correct encodings for umlauts
9) run the Ruby crack program
While I'm almost illiterate in German I'm very familiar with the computer-specific problems encountered by German-speaking people, especially in multi-language places like Switzerland. Because of the QWERTY vs. QWERTZ vs. AZERTY keyboard layout issue, when you were entering your password sight-unseen you may have entered some other characters because of accidental switching of the keyboard layouts. Have you actually verified in the Language Bar that you were using the correct layout while typing your password?
Does it mean that it is possible that if I created a wallet password in a PC which crashed, now in a new PC maybe Bitcoin-qt does not accept the password?
If this is the case, and I don't know the encoding of regedit.exe, what can I do to be sure that I test all the encoding possibilities in the ubuntu's terminal?