What does the "The View State is invalid ..." error message means?

What does the "The View State is invalid ..." error message means?

  
You may receive the "The View State is invalid for this page and might be corrupted" error message under the following circumstances:

Scenario 1: You recompile your oml. A user has a browser opened with a copy of the page that was requested before this change, and the user posts the page after the files in that folder were regenerated. -and/or-

Scenario 2: Your ASP.NET application is hosted on a Web farm (you have several Hub Nodes) and you have not run the Hub Server Configuration, along all Hub Nodes, with the same server.hsconf file. -and/or-

Scenario 3: Your ASP.NET application is hosted on a Web farm (you have several Hub Nodes). There are accesses to the application using different case sensitiveness in URLs.

NOTE: In Scenario 2, assure that the machine keys match for all of the computers in the Web farm. The Hub Server Configuration application will rewrite this key in machine.config. To assure the key is the same, you must for each Hub Node:
Copy server.hsconf file from Controller Node to each Hub Node (by default, directory 'C:\Program Files\OutSystems\Hub Server').
Run [Start Programs]\OutSystems Hub Edition\Configure Hub Server
Click Ok and answer Yes if asked to restart services.

NOTE: Scenario 2 is an expected behavior that occurs when the Screen was changed. If the user restarts the session (rewrites the application URL) will solve the problem.