Hello Teviot,
I'm sorry, but "can't log in" is too big a problem space for us. First of all, there is no requirement to log-in to FOP, you just click the button and the FOP screen appears. There is no challenge response unless you attempt to have FOP make a call for you. So... "can't log in" is, itself, a puzzling statement on the face of it. However, you need to give us some more clues... What state is the screen in after the attempt? What does the SME server message log have to say on the subject (if anything).
These things we need to know to help you. FOP has been present almost from the first in SAIL and we've never had anyone report a "log-in" type fault with it before.
Best
S