I have this same problem!
I had thought it was my service packs on the server, but I read this post and tested on my custom 404.asp page. Sure enough - that is the problem.
The page that I am trying to set the editor up on does not hit the 404.asp page at all. The editor itself must be pulling something from the server in the background.
I am using the Cute Editor for ASP 5.0.
My errors are also syntax errors but my line numbers change when I change my 404.asp page. ( line 4 or line 7..)
Please shed some light on this problem!
Jessie