Seemingly random invalid viewstate errors on same page

This error appears sporadically on our website, on the same page (and this page only!). I do not believe the IP to be a Google bot.

Does anyone have any idea what might be throwing this error? I can browse the web page just fine from multiple devices, computers and regions.

URL: https://www.[REDACTED]/contact
Physical Path: [REDACTED]pages/contact.aspx

System.Web.HttpException (0x80004005): The state information is invalid for this page and might be corrupted. ---> System.Web.UI.ViewStateException: Invalid viewstate. 
    Client IP: [REDACTED]
    Port: 12984
    Referer: http://www.google.com
    Path: /pages/contact.aspx
    User-Agent:  [REDACTED]
    ViewState: eIwg1qr94M/Pt7WX7mVEmEL6qUQefj5oyrN41gHviLmfOkK3I 55Tf12OzxFR4p784tWyD7/FJM4Hh5LHDcsYbUjbRs= ---> System.FormatException: Invalid length for a Base-64 char array or string.
   at System.Convert.FromBase64_Decode(Char* startInputPtr, Int32 inputLength, Byte* startDestPtr, Int32 destLength)
   at System.Convert.FromBase64CharPtr(Char* inputPtr, Int32 inputLength)
   at System.Convert.FromBase64String(String s)
   at System.Web.UI.ObjectStateFormatter.Deserialize(String inputString, Purpose purpose)
   at System.Web.UI.Util.DeserializeWithAssert(IStateFormatter2 formatter, String serializedState, Purpose purpose)
   at System.Web.UI.HiddenFieldPageStatePersister.Load()
   --- End of inner exception stack trace ---
   at System.Web.UI.ViewStateException.ThrowError(Exception inner, String persistedState, String errorPageMessage, Boolean macValidationError)
   at System.Web.UI.HiddenFieldPageStatePersister.Load()
   at System.Web.UI.Page.LoadPageStateFromPersistenceMedium()
   at System.Web.UI.Page.LoadAllState()
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
   at System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
   at System.Web.UI.Page.ProcessRequest()
   at System.Web.UI.Page.ProcessRequest(HttpContext context)
   at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.<>c__DisplayClass285_0.<ExecuteStepImpl>b__0()
   at System.Web.HttpApplication.ExecuteStepImpl(IExecutionStep step)
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

System.Web.UI.ViewStateException: Invalid viewstate. 
    Client IP: [REDACTED]
    Port: 12984
    Referer: http://www.google.com
    Path: /pages/contact.aspx
    User-Agent: [REDACTED]
    ViewState: eIwg1qr94M/Pt7WX7mVEmEL6qUQefj5oyrN41gHviLmfOkK3I 55Tf12OzxFR4p784tWyD7/FJM4Hh5LHDcsYbUjbRs= ---> System.FormatException: Invalid length for a Base-64 char array or string.
   at System.Convert.FromBase64_Decode(Char* startInputPtr, Int32 inputLength, Byte* startDestPtr, Int32 destLength)
   at System.Convert.FromBase64CharPtr(Char* inputPtr, Int32 inputLength)
   at System.Convert.FromBase64String(String s)
   at System.Web.UI.ObjectStateFormatter.Deserialize(String inputString, Purpose purpose)
   at System.Web.UI.Util.DeserializeWithAssert(IStateFormatter2 formatter, String serializedState, Purpose purpose)
   at System.Web.UI.HiddenFieldPageStatePersister.Load()
   --- End of inner exception stack trace ---