So we have an existing web app with regular logins and want to add availability to use their Microsoft account.

Tested with a test-app there were no problems, and I did not specify a machinekey that looks like it's needed now.

Any suggestions? Full error message when the Microsoft Login-button is clicked:

The anti-forgery token could not be decrypted. If this application is hosted by a Web Farm or cluster, ensure that all machines are running the same version of ASP.NET Web Pages and that the <machineKey> configuration specifies explicit encryption and validation keys AutoGenerate cannot be used in a cluster.