I'm running two 4.8.0 sites off the same server.

I recieved this error when I attempted to login to one of the sites today:

DotNetNuke.Services.Exceptions.PageLoadException: Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that <machineKey> configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster. ---> System.Web.HttpException: Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that <machineKey> configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster. ---> System.Web.UI.ViewStateException: Invalid viewstate. Client IP: 207.34.78.50 Port: 12370 User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1.14) Gecko/20080404 Firefox/2.0.0.14 ViewState: 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... ---> System.Web.HttpException: Unable to validate data. at System.Web.Configuration.MachineKeySection.GetDecodedData(Byte[] buf, Byte[] modifier, Int32 start, Int32 length, Int32& dataLength) at System.Web.UI.ObjectStateFormatter.Deserialize(String inputString) --- End of inner exception stack trace --- --- End of inner exception stack trace --- at System.Web.UI.ViewStateException.ThrowError(Exception inner, String persistedState, String errorPageMessage, Boolean macValidationError) 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() at System.Web.UI.Page.LoadPageStateFromPersistenceMedium() at System.Web.UI.Page.LoadAllState() at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) --- End of inner exception stack trace ---

I logged into the other site and received the same error. I then tried to login again and the error disappeared (same for the other site). So there is one instance of the error in the event logs for each site. The sites appear to be functioning properly now. They have been functioning properly up to this point and I've never encountered this error before. I double checked the two machine keys and they are not the same. Has this happened to anyone before? Is there a configuration issue I need to deal with?

Thanks!