I recently got an issue where customer ASP.NET 2.0 website on IIS and on one of the aspx pages he was using RangeValidator control. Browsing to that aspx page was giving client side Java script errors ‘Webform_Postbackoptions is undefined’. Without RangeValidator application was just working fine.
WebResources.axd was not getting downloaded to ‘Temporary Internet Files’ folder. Fiddler showed 500 Status for the WebResources.axd. IIS Logs showed nothing in my case. When directly browsed to this file it gave errors in one of the events called AcquireRequestState in Global.asax file sitting under App_Code directory. Error was related to session state because an AXD file doesn’t have any session state handler associated.
Commented out AcquireRequestState event and now webresources.axd is successfully downloading in ‘Temporary Internet Files’ folder.
Note:- Fiddler tool can be downloaded from http://www.fiddlertool.com/dl/FiddlerSetup.exe. It's a great tool to troubleshoot client side issues.