Invalid length for a Base-64 char array - System.Web.UI.ViewStateException: Invalid viewstate

Question

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What causing this “Invalid length for a Base-64 char array”

This error has been in our logs from time to time and various users experience it:

System.FormatException: Invalid length for a Base-64 char array.
    System.Web.HttpException: The client disconnected. ---> System.Web.UI.ViewStateException: Invalid viewstate.
        ViewState: 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... ---> System.FormatException: Invalid length for a Base-64 char array.

One user reported to have experienced the error when he was searching for customer details, which normally pulls data from a web service. He noticed that the list wasn't being generated and kept on pressing search and at the third time, he got our global error message. I checked the logs and got the one above.

Is it because the viewstate timed out? Anyone have an idea on this?

Thanks!