Question : 500 ORT not reconised

A client of mine is having issues connecting to a server to upload his files.
Up until 2 weeks ago everything went fine..

now often time his files start to transfer but are slow then after 2 or 3..the file hangs and times out

we get  the following error:

 500 ORT not understood

I can't seem to find ANYWHERE that tells me what that means...

He can connect to other servers no problem and transfer, its only this one server giving the problems.
I can connect from my location using the same ftp program and have no problem.

We used another ftp program with him and still have issues transfering.

Hes running a Win XP system
Linksys bef11s4 router. (I even have upnp enabled and have tried him on DMZ)

we have tried using Cute ftp Pro 6.0 purchased.
and WS FTP LE.

can anyone suggest anything?

Answer : 500 ORT not reconised

If you perform a tracert to the IP address from his machine, does the route differ than the one you see on the machines that work?