Bear in mind I was already over my head with my prior post. Just trying to think of places where you can look to investigate.
Even without access to the other server, check the httpd.conf or httpd.include or .htaccess files for the two Symlinks settings described previously. The FPE may require that one of them be enabled. It's something to experiment with. That is the solution to one particular FP problem that I'm aware of, but it's not the one you are describing.
The various FrontPage errors can have a variety of causes. One error message does not map to exactly one cause. So it's unlikely anyone can post here with, "This will be the problem..." I know I can't. It is probably going to involve some web searching and experimentation. The thing to search on is (with the quotes):
FrontPage "security violation"
If (but only if) the client doesn't use other FP features that require the FPE's, you could eliminate the problem by uninstalling the FPE and publishing with FrontPage FTP instead. That's what I do, and it works well. FTP publishing is slower than HTTP, though; especially noticeable on dial-up.
What I was trying to get at with the comments about httpd.conf etc. is that just because both servers use WHM/cPanel doesn't mean they are configured the same. They could be quite different, and configuring the VPS adds yet more complexity.
The client should not be trying to use the same password as they were using before. The reason for changing the pw to give the Extensions a "kick" is that if the cPanel and FrontPage passwords have gone out of sync, a pw change can get them in agreement again.
--> Try doing it this way: uninstall the FPE, and "clean up" old extensions. Then change the password (wherever you do it, WHM/cPanel/other login?), then install the FPE again. That is even more likely to force the cPanel password to be registered properly with the FP Extensions.
Another thing that can cause FP publish errors is Apache mod_security. If the pw change above doesn't solve the problem, try putting this code in the top-level .htaccess:
[edited by: SteveWh at 11:58 pm (utc) on Mar. 1, 2008]