command line can't be entered

I can't open the command line in minecraft.
It doesn't matter what key I assign the action to (tested it with T, Q, Numbers, NumPad Numbers, Shift, Return, ... - yes, I did assign the default actions of other keys to other values so that there is no conflict) it can't open the command line.
Chat has the same problem. I tested both in multiplayer with the same result.
The keyboard is not broken nor does Minecraft can't detect the keyboard hit. If eg. drop item is bound to T it works fine.