Unable to validate data, EncryptOrDecryptData problem

I am getting hundreds of these errrors on the production server (IIS7.5, .net 4.5 web app):
Exception type: System.Web.HttpException Exception message: Unable to validate data. Request URL: http://domainname/WebResource.axd?d=vIkcOYaxqSd6eMCUlxDkdQVxjxcaRV8fWPLFaNn8W4w1&t=633802992069218315&AspxAutoDetectCookieSupport=1 Request path: /WebResource.axd Stack trace: at System.Web.Configuration.MachineKeySection.EncryptOrDecryptData(Boolean fEncrypt, Byte[] buf, Byte[] modifier, Int32 start, Int32 length, Boolean useValidationSymAlgo, Boolean useLegacyMode, IVType ivType, Boolean signData)
They began after I migrated the site from .net 3.5 to .net 4, then 4.5. This is the machineKey element in web.config:
<machineKey validationKey="KeyA" decryptionKey="KeyB" validation= ...