This is what is happening to me as far as I can tell. It has to do with certain servers being accessable and certain servers not being accessable.
An example that just happened: I was in Whitegate (I managed to log in fine for once), and I took the runic portal to Caedarva Mire. As soon as I hit the "Downloading Data" screen I get the POL-3001 error. (well, after it times out)
Apparently, for Diabolos, the POL server for Caedarva Mire is 219.117.114.155 (using the vista Resource Monitor application). It tries to connect but can't.
And it's independant of any windower meddling. I can't ping that IP address, however many other people can (the zone isn't down, just unreachable for me at my current ISP). Traceroute to 219.117.114.155 gets as far as 220.148.131.120, and every step after that point is dead. (that's not my ISP's router, it's some t-com.ne.jp router). This is the important fact here; It's not your ISP that is to blame - it's some place further down the line as far as I can tell. (EDIT: I'm stupid, traceroute won't work with a UDP connection, ignore this paragraph. Still, there's some weird mojo in the internets tonight).
I talked to a GM yesterday night about this and apparently many people are getting this error. They said for us to contact our ISP; And that if contacting your ISP fails, to contact SE during their normal business hours Mon-Fri.
In the meantime, however, I can't log on with any regularity. Sometimes, the internet routers work, and other times they don't. Depends which zone server you need to talk to. For instance, I have 1 mule in each of the starter cities, and my main character was in Windurst Woods. I could log on just fine to my other mules, but both my mule in Windy Woods and my main character in Windy Woods would trigger the POL-3001 error.
XI Wiki

