A user was trying to download a note attachment when the message "Invalid Action" just appeared in the browser.
The first thing i did was using my old friend Diag Tool where i saw the exception "The key specified to compute a hash value is expired, only active keys are valid.", i have found as well that in the Event Viewer.
Doing some research i saw that restarting the MS CRM Asynchronous Processing Service or running the executable from the Tools folder inside CRM folder Microsoft.Crm.Tools.WRPCKeyRenewal.exe /R, i could solve the problem.
I did that, i restarted the server and in the end, the same problem. What do you do in this situations, just google it a little bit more, so, after 5 minutes someone had the same issue, and that one more step neededto be done, Clear the Browser Data on the client side was the solution.
Hope it helps.