The same thing happened to me just this past week. I'd log into a zone with my character and it'd start to load, then just stop and I'd get kicked with POL Error: 2003 (I think that was it) Failure to Load
And then I just couldn't get on my character, it'd begin to load and stop, though I could log onto my mule just fine. I'd have a friend of mine who has access to my account to "save" me by zoning me back to areas that still worked for me.
Basically, your files are corrupt. I ultimately ended up having to completely wipe my computer and reinstall everything, takes awhile.
"Unable to enter the next area. Expansion Pack not installed"
This also irked me. You can't just unistall/reinstall basic FFXI (w/ no expansions) or POL. If you unistall FFXI, you'll have to reinstall all the expansions. Although I was able to unistall/reinstall CoP and ToAU independently.
So, good luck with that.
Btw, I run on Vista (which I blame for all my troubles). Are you as well? That might have something to do with it.
Edit: It wasn't just FFXI I was having problems with either, just wanted to clarify that, it was my entire PC, FFXI was just affected as well. Edited, Feb 1st 2008 2:14am by Pokiehl