Comment # 3 on bug 1081676 from Martin Vidner

I think it is not specific to fast_gettext. It's rubygems themselves, and it
may be a generic Ruby bug.

Johannes, can you, as that user, run

    xdg-su -c "ruby -r fast_gettext -e 'puts :it_works'"

to see if you get the error too? If yes, please run 

    xdg-su -c "strace -o /tmp/strace.log ruby -r fast_gettext -e 'puts
:it_works'"

and attach the log.