Hi Thierry,
AFAIK only user names starting with a number are really forbitten and the user name is not the problem. It seems, that tdm dosn't like the Umlaut in the password.
Have you tried setting the password in kuser? In my opinion, if kuser lets you set a password but does not accept later, it would be a bug.
No. I use the unix/linux program passwd.
Fine regards Rolf