thanks for the quick reply. I've tried it both as root and normal user, same result.

Here the other outputs:
~ # ls -l /usr/bin | grep gem
lrwxrwxrwx 1 1000 1000 17 Jan 17 17:53 gem -> /opt/ruby/bin/gem

~ # ls -l /opt/ruby/bin
-rwxr-xr-x 1 1000 1000 547 Jan 11 02:27 gem

I've now uninstalled the ruby-gems package and installed ruby-gems from rubyforge, this works fine.

Not sure if it's my system or the package.

Cheers,

Fabian