WebResource.axd throwing exception

Question

I have an issue with my ASP.Net 3.5 web forms site where sometimes requests to:

WebResource.axd?d=eIozr3loXr3APyEewE38BIDKqN_q3qvb1YMpcivEzsc8xk5zz-o6uaPkpyFICfz_9GLwnEFNScd7yKWROBOvLU4gE-Y1&t=634604893351482412

results in a exception thrown. This is the request to load in the WebForms post back javascript (e.g WebForm_DoPostBackWithOptions, WebForm_DoCallback, etc.).

The error seems very similar to this SO post: WebResource.axd throwing exceptions in web farm, with the exception that the production environment is a single server, not a web farm. This rules out any issues with differing machine key elements across different servers, or different System.Web.dll versions.

What other circumstances could result in this problem sporadically occurring?

Edit: I've just noticed that failing requests are actually coming from:

webresource.axd?d=73uzdnkyw4tt5-xgrmheyelsy01btqexhl8hk8myqspxxbu6krzyliy7e8-6jyofqge7m6uwfve9be3rhinncnv94v01&t=634258886654344891

Which is all lower case, which will be causing the Base64 decoding to fail. Not sure why sometimes the aspx page is spitting out this link in lower case however.

Answer 1

Turns out there was an Http Module that was redirecting the URL with a .ToLower(), this meant that the Base 64 encoded query string in WebResource.axd was modified therefore failed parsing.