Cataclysm doesn't like TAB as a keymap?

I mapped ‘open inventory’ to the TAB key and it actually works fine… at least until I restart the game.
I get debug warnings upon start up telling me about multiple keys being mapped twice.
The keys are:
e (inventory ?!), a (apply), d (drop), e (examine), f (fire), m (map), o (open), s (smash).

Ingame options reveals that a bunch of keys are suddenly assigned to ‘open inventory’.

The keymap.txt itself looks fine, though. No mulitple key assignments.