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.Util.DeserializeWithAssert(IStateFormatter formatter, String serializedState) at System.Web.UI.HiddenFieldPageStatePersister.Load()
I'm getting that error in my logs.
The page throwing the error has a query string variable which value is UrlEncoded (greek characters encoded in % characters).
The error is not throwing always.
What's the cause of throwing that error?
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