Tou may find the problem is not that you can't type, but rather that the writing in the PMs is actually white (on a white background). This means when when you do type something it is there but you just can't see it! Try typing something and then highlight the area where the writing should be to see if that's you're problem. I think we all have the same problem since the boards background colour changed from black to white. You certainly have to highlight the blank area in PMs received if you want to read them. The other way around it is to type in a different font colour.
There have been a couple of posts about this since the change but so far no fix. Hopefully Don will get to the bottom of it soon.