My colleague, Michael Knudsen, has been struggling with a customer & partner for several hours now with an annoying problem on an EPiServer 4.62 site just moved to a production server. Finally, after browsing through half the web and getting valuable feedback from most supporters / developers known to man, a simple solution was discovered.
To save other people the agony he's gone through he persuaded me to post the solution here...
If you don't know what webresources does there's a good intro here. This problem had us all stunned for a while - lots of theories going through our heads...."It must be a web.config problem - some handlers are interfering with each others", "Perhaps asp.net isn't properly installed on the server", "But it's working on my machine", "Is this using AJAX?", "Can't we just put the dev. server where it works into production", etc. were some of the thoughts going through our heads as the problem took over our common sense.
Uncheck this checkbox in IIS: