[SOLVED] Comment form invisible

In one of our sites the comment form just won't show up. Or rather: it IS there, but the normally white writing box won't show up - as you see here.
I have used the exact same translated comments.php file as here - and also copy-pasted all Atahualpa/Comments settings to avoid problems.
Still can't figure out what's wrong. The funny thing is that it IS possible to write in the invisible field, so it could be some CSS trouble. But - as said - I have made everything as equal as possible in the two sites.
Any ideas?

PS - The "two sites" are at the same domain but is built with two WP installations.

Thanks,