error 225 problem - please help

Since 2 days I cannot log in to GW and get following message:

"You may not login from this location because this Internet address (IP address) is believed to be running open proxy or relay. You must connect directly to Guild Wars. Please see http://www.guildwars.com/support/en-open-proxy.php for further information. (code=225)"

The point is that between successful login and first failed one nothing changed on my router nor on my PC. Just one day I started getting this error out of the blue. After long discussion with support they point me to my router as a possible issue (I can access GW when connecting directly). However I did not touch my router for like 2 years and then suddenly this appears...

In router settings today I put my IP to be directly connectible to Internet so that at least one PC can play GW. I made sure that port 6112 is open... still nothing. Any tips or suggestions?

My router is Dl-524

thank you for your time.

EDIT: regarding the suggestions in the link of the message. I did not change anything in my system so it just could not by itself become an open proxy. For the last two days I did all possible virus scans including the programs available from antimalware toolkit - nothing found. IP also seem not to be blacklisted on the NJABL

By this..what do you mean? Do you mean if you physically bypass the router with your PC's ethernet cord straight to your modem, you can connect? If that's the case...the problem does not lie with your ISP, your public IP or anything like that.. it is indeed your router. Have you tried doing a full factory default hard-reset on your router at all?

Have you tried resetting your router? Or unplugging the power to the router for a few seconds and plugging it back in?

Yes. When connected by ethernet cable to modem then I can connect.

Yes and even changed its IP to no avail.

Yes did it a few times.

Fortunately issue seems to be solved now.

It seems the issue was with the WAN IP (not sure it was mine from ISP or ISP one). The standard internal one for router I could change (192.178....) but that did not help. It appeared that the IP visible in WAN section of the router was the problem. No idea if this was IP I got from ISP or was it IP to which I was connecting to at ISP. Anyway resetting router did not change it and resetting modem either. However this IP was indeed blacklisted. I removed it from the list.

Thank you for your help so far. I just have one more question to be sure how to avoid this problem again. In the WAN section of the router IP which is there is it ISP IP or is it the IP router gets from ISP? In other words is it the IP to which I connect or with which I connect?