I'm running SQL 2012 on Windows 2012
Recently users reported an issue when trying to change the schedule for certain Report Subscriptions.
They can edit the subscription but when they try to access the Schedule they get the error
Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that <machineKey> configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster. http://go.microsoft.com/fwlink/?LinkID=314055
The problem only affects some report subscriptions (always the same reports).
Most report subscriptions do not get the problem and the schedule can be changed.
Copying the report to another folder does not fix the issue.
The permissions on all the folders appear to be the same.