Ok, heres' what I have found, all with the /login.aspx Url
if I do a GET, the url works OK
if I do a POST, the url fails with a 404
If I do a POST and take out the Request Body, the URL works OK
if I do a GET and put in a Request Body, the url fails with a 404
Conclusion : It is the inclusion of a Request Body with the request that causes the 404 error.
I am guessing this is to do with the NeatUpload component, which monitors the request body in order to provide the upload progress bar.
Two things to try:
1) comment out the NeatUpload component in the web.config file and see if that fixes it
2) try adding this attribute (to the add name='ifinity.urlmaster'...) line in the web.config: allowDebugCode="false"