That could be the problem, although how on earth you have that many is beyond my understanding.
As a moderator I get this:
Inbox contains 135 messages.
You have 377 messages stored, of a total 500 allowed.
I would imagine that that is equivalent to a Full Member's allowance.
Might I suggest you download your PMs, if you haven't tried it before, do it in all three available formats, then start deleting whole pages worth of your sent PMs from your earlier time on here.
Get your total down under 500 and see what happens.