I just found this same issue in two sites (separate installs) that had been upgraded over the years from 4.x to 6.29 and then to 7.3.2 just this week. It is browser dependent affecting IE 11 but not Chrome or Firefox. The following exception is being logged and displayed to super users:
AssemblyVersion:7.3.2
PortalID:-1
PortalName:
UserID:-1
UserName:
ActiveTabID:-1
ActiveTabName:
RawURL:/WebResource.axd?d=S52A8QTw_Lg0CpwoVnIDCz6JZhCHoBM4axhIu-AmRLYPVxHCoY-OvI_6uJS1Snjt0z9yvdM1h3EaLWegjtJ3-D9-otxQYE9GLPV2lh3LlyaxY0m8NZ748Y0hxvni_8bfbDpUI23Vf5kaYfz6-ewSMxacTykzX4sIOZ4bDw2&t=634783790348009208
AbsoluteURL:/WebResource.axd
AbsoluteURLReferrer:http://www.bridgtonalliancechurch.org/DesktopModules/Admin/RadEditorProvider/Css/EditorOverride.css
UserAgent:Mozilla/5.0 (Windows NT 6.1; WOW64; rv:29.0) Gecko/20100101 Firefox/29.0
DefaultDataProvider:DotNetNuke.Data.SqlDataProvider, DotNetNuke
ExceptionGUID:d7551d5e-0f46-442a-b004-3c90ac5a9e7d
InnerException:This is an invalid webresource request.
FileName:
FileLineNumber:0
FileColumnNumber:0
Method:System.Web.Handlers.AssemblyResourceLoader.System.Web.IHttpHandler.ProcessRequest
StackTrace:
Message:
DotNetNuke.Services.Exceptions.PageLoadException: This is an invalid webresource request. ---> System.Web.HttpException: This is an invalid webresource request.
at System.Web.Handlers.AssemblyResourceLoader.System.Web.IHttpHandler.ProcessRequest(HttpContext context)
at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)
--- End of inner exception stack trace ---
In IE11, I'm also getting the JS error showing in Console when opening the Image Manager:
'Sys' is undefined at DialogHandler.aspx, Line:40, Column:1 ----
Sys.WebForms.PageRequestManager._initialize('RadScriptManager1', 'Form1', [], [], [], 90, '');