how should machineKey be formatted in .net 4.5 webforms?

I recently migrated a web application from .net 3.5 to .net 4.5. The machineKey element is:

<machineKey validationKey="KeyA" decryptionKey="KeyB" validation="SHA1" decryption="3DES" compatibilityMode="Framework20SP1" />

I am getting hundreds of these errors:

Exception type: System.Web.HttpException
Exception message: Unable to validate data.
Request URL: http://imap.accoutrement.com.au/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)
at System.Web.UI.Page.DecryptString(String s, Purpose purpose)

ViewSource suggests the culprit could be an asp:Menu and/or viewstate.

Searching for a solution suggests machineKey could be the problem.

Thanks for any help.