Then we'll need Dange to have a look at it. As far as my knowledge goes, it is highly unlikely for the client to do this in the first place. I would say impossible, but I can't say that for sure.
A bug like this also sounds highly unlikely to me, because the client is supposed to send whatever you type in, and not tamper with it in any other way than encrypt it. One slight possibility is that something is blocking the data sent, but only partly, so that the username and password that the server receives are messed up. Again, it might not even be possible.
Bugs usually won't occur consistently like this either when it's a critical function that has worked perfectly for years.
So my suggestion is this:
You misspelled the password upon registration and misspelled again upon logging in to the forums. Have you logged out and in to the forums since, so that you have to type the password several times?