I have this problem
all the time. I wasn't able to get on there for the past several WEEKS, as a matter of fact. Last night I cleared my browser history, cleared my browser cache, cleared my cookies, set my browser to accept all cookies, cleared all saved passwords and auto-fill info.... and STILL wasn't able to get on, despite trying about thirty times. No joke.
However...
This morning, I just clicked, typed my info, and... voila! I got on. Now, that's the good news.
The bad news is that it appears that Disney hasn't updated the Passholder site yet to reflect the latest round of AP discounts that came out yesterday (or was it the day before?).
I used to access the AP site all the time, with no problems. Then, about a year ago, it started acting all wonky.

Like I said, sometimes weeks/months go by when I can't get in there at all.
I probably haven't helped you (though you may want to try all the stuff I listed in the first paragraph), but at least you know you're not alone!!
-dannie