Having same problem as this person here (I responded to this thread but it was apparently deleted without anyone contacting me)
What's happening is that the IBM WebSphere "plugin" (an ISAPI filter) seems to be grabbing requests before your filter.
According to Microsoft, even if your plugin is given higher priority, if the other filter keys off an earlier event, it will be loaded first. In this case, IBM picks the URL up on the SF_NOTIFY_PREPROC_HEADERS event. Your plugin (and most other rewriters) seems
to be picking the event hook of SF_NOTIFY_URL_MAP.
In this case, IBM takes the request, and forwards it off to WebSphere, before your rewriter can get a chance to do it's magic, since the URL seems to get picked up as the DLL name of the IBM filter, instead of the original path.
I'm a Java guy so I'm very open to your suggestions. And I would be very willing to test a new compiled version that loads up on SF_NOTIFY_PREPROC_HEADERS if you would be interested in compiling one. I saw you have commented code in your source that seems to
be a hook for SF_NOTIFY_PREPROC_HEADERS.
I may try to compile this new version myself but I would love some help and your opinion on if this is an OK solution.