I had to do a system restore to fix my issue with this. Wasn't able to find any other fix. Happened a lot in GC and RF. I no longer play for now. May return soon. Looking in ti other games with not so much p2w items needed to actually build a decent char. Anyways gl.
That's not a SO error, you don't need to format or downgrade to Wxp. Could be 2 things: The cheksum found changes in SCR_BASE.XP or ENVIROMENT.XP or the HTTP request to a playredfox host to verify the integrity of the files with a hash didn't reach its destiny. Try redownloading the client and install again, if the error persists then something is blocking the http cheksum verification; a firewall or antivirus may be the problem.