After upgrading ASP .Net 3.5 app to .NET 4.5.2 we started receiving numerous “This is an invalid webresource request” errors.
We have traced the error to missing css from Obout control dll. The control is used widely and we cannot replace it.
The upgrade was performed month and half ago and we still receive the same number of errors.
I am aware of other Stackoverflow questions regarding the same problem, but some of the answers suggest problem disappears with time. This has not happened in our case yet, almost 2 months after upgrade.
Not really sure how that happened but try adding this code to your web config
And then in your IIS, under Machine Key for the website, we have Encryption Method = SHA1 Decryption Method = DES
make sure all the checkboxes for Validation key and Decryption key are unchecked. Note that "xxx" and "yyy" are in the key fields.
After making this change, you may need to recycle your app pool, and clear browser caches.