System.FormatException: Invalid length for a Base-64 char array

hello,
i have been receiving a number of these errors in my web application:
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System.Web.HttpException (0x80004005): The state information is invalid for this page and might be corrupted. ---> System.Web.UI.ViewStateException: Invalid viewstate. Client IP: xxxx Port: xxxx Referer: http://www.google.com/url?url=<domain> Path: /WebForms/Privacy.aspx User-Agent: Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; WOW64; Trident/6.0) ViewState: 3ieXgi...NhA1s= ---> System.FormatException: Invalid length for a Base-64 char array. at System.Convert.FromBase64String(String s) at System.Web.UI.ObjectStateFormatter.Deserialize(String inputString) at System.Web.UI.ObjectStateFormatter.System.Web.UI.IStateFormatter.Deserialize(String serializedState) at System.Web.UI.U ...