This is a problem that's been plaguing me for days now and I just managed to get around it. I'm posting this just in case someone has the same issue and for the admins to know.
I tried using the same password I use on the forum and kept getting that damn error. Tried different browsers, same story. I like solving problems on my own, so I didn't want to ask for help.
I was running out of ideas, until I thought "Could it be that my forum password is too long? Or perhaps that it has a < character?". (I tend to use long and complicated passwords, truth be told)
I tried removing 2 characters, including that symbol and... done!
If it was the symbol that did it, could the admins perhaps restrict forum passwords from using it just to save other people the headache?