The difference between “.NET Error Pages” and “Error Pages” in IIS7

After searching for more combinations of IIS and plus, it appears that IIS7[.5] is set up to reject URLs with a plus sign by default out of some fear of the use of that character; that symbol is still allowed in the querystring, though. The solution is to alter the requestFiltering attribute default on <system><webServer><security><requestFiltering> to allow doubly-encoded characters with a command line call (ultimately modifying your ASP.NET web.config):

%windir%\system32\inetsrv\appcmd set config "Default Web Site" -section:system.webServer/security/requestFiltering -allowDoubleEscaping:true

This may be a bit more dangerous than one prefers to be with their web site, but there didn't appear to be a way to be more specific than a blanket allow. The warnings were regarding the mismatching that could occur between using a plus in a URL and its typical translation as a space. It looks like the only other alternative is to stop using plus characters in your URLs at all.

After finally getting the correct rule set up on DebugDiag so that I could get a dump for this, I was able to pinpoint the problem - there was a versioning error between the ASP.NET pages I had deployed and a COM+ module that they use, and it was causing an unhandled exception that then caused an infinite loop of unhandled exceptions when it tried to log the error, thus crashing my app pool.