Thanks Sue.

The change was made via the web front end. Someone made a change to the SQL script in the data driven subscription.

The stack dump above is from the Portal.WebHost log. Nothing preceding it is related. Here's what's in the reporting services log from around the same time. We don't use PowerBI on our report server so I think that is a red herring.

library!ReportServer_0-1!3018!01/09/2018-14:54:08:: i INFO: Call to GetReportParametersAction(/<reportPath>). User: xxx.
library!ReportServer_0-1!e48!01/09/2018-14:54:08:: i INFO: Call to GetItemTypeAction(/<reportPath>). User: xxx.
library!ReportServer_0-1!e48!01/09/2018-14:54:08:: i INFO: Call to GetReportParametersAction(/<reportPath>). User: xxx.
library!ReportServer_0-1!e48!01/09/2018-14:54:08:: i INFO: Call to GetItemTypeAction(/<reportPath>). User: xxx.
library!ReportServer_0-1!e48!01/09/2018-14:54:08:: i INFO: Call to GetReportParametersAction(/<reportPath>). User: xxx.
library!ReportServer_0-1!e48!01/09/2018-14:54:08:: i INFO: Call to GetItemTypeAction(/<reportPath>). User: xxx.
library!ReportServer_0-1!e48!01/09/2018-14:54:08:: i INFO: Call to GetReportParametersAction(/<reportPath>). User: xxx.
library!ReportServer_0-1!e48!01/09/2018-14:54:08:: e ERROR: Throwing Microsoft.ReportingServices.Diagnostics.Utilities.NotEnabledException: , Microsoft.ReportingServices.Diagnostics.Utilities.NotEnabledException: The requested functionality is not currently enabled.;
extensionfactory!ReportServer_0-1!e48!01/09/2018-14:54:08:: i INFO: Skipped instantiating Report Server PowerBI report server extension. Extension was not enabled.
library!ReportServer_0-1!e48!01/09/2018-14:54:08:: e ERROR: Throwing Microsoft.ReportingServices.Diagnostics.Utilities.OperationNotSupportedNativeModeException: , Microsoft.ReportingServices.Diagnostics.Utilities.OperationNotSupportedNativeModeException: This operation is not supported on a report server that runs in native mode.;
library!ReportServer_0-1!e48!01/09/2018-14:54:14:: e ERROR: Throwing Microsoft.ReportingServices.Diagnostics.Utilities.NotEnabledException: , Microsoft.ReportingServices.Diagnostics.Utilities.NotEnabledException: The requested functionality is not currently enabled.;
extensionfactory!ReportServer_0-1!e48!01/09/2018-14:54:14:: i INFO: Skipped instantiating Report Server PowerBI report server extension. Extension was not enabled.
library!ReportServer_0-1!e48!01/09/2018-14:54:14:: e ERROR: Throwing Microsoft.ReportingServices.Diagnostics.Utilities.NotEnabledException: , Microsoft.ReportingServices.Diagnostics.Utilities.NotEnabledException: The requested functionality is not currently enabled.;
extensionfactory!ReportServer_0-1!e48!01/09/2018-14:54:14:: i INFO: Skipped instantiating Report Server PowerBI report server extension. Extension was not enabled.
library!ReportServer_0-1!3464!01/09/2018-14:54:48:: i INFO: Call to GetItemTypeAction(/<PathToSomeOtherReport>). User: xxx.
library!ReportServer_0-1!3464!01/09/2018-14:54:48:: e ERROR: Error rending control: System.Web.HttpException (0x80004005): 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.

See http://go.microsoft.com/fwlink/?LinkID=314055 for more information. ---> System.Web.UI.ViewStateException: Invalid viewstate.
    Client IP: xxx.xxx.xxx.xxx
    Port: 50050
    Referer: https://reports.exemplarhs.com/ReportServer/Pages/ReportViewer.aspx?%2FSan%20Bernardino%2FProductivity%20Pilot&rc:showbackbutton=true
    Path: /ReportServer/Pages/ReportViewer.aspx
    User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/63.0.3239.132 Safari/537.36
    ViewState: /<some sort of security key or token>
 at System.Web.UI.ViewStateException.ThrowError(Exception inner, String persistedState, String errorPageMessage, Boolean macValidationError)
 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()
 at System.Web.UI.Page.LoadPageStateFromPersistenceMedium()
 at System.Web.UI.Page.LoadAllState()
 at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
library!ReportServer_0-1!3464!01/09/2018-14:54:48:: e ERROR: Throwing Microsoft.ReportingServices.Diagnostics.Utilities.ServerConfigurationErrorException: Could not validate Viewstate., Microsoft.ReportingServices.Diagnostics.Utilities.ServerConfigurationErrorException: The report server has encountered a configuration error. ---> System.Web.UI.ViewStateException: Invalid viewstate.
    Client IP: 172.16.27.113
    Port: 50050
    Referer: https://reports.exemplarhs.com/ReportServer/Pages/ReportViewer.aspx?%2FSan%20Bernardino%2FProductivity%20Pilot&rc:showbackbutton=true
    Path: /ReportServer/Pages/ReportViewer.aspx
    User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/63.0.3239.132 Safari/537.36
    ViewState: /
<some sort of security key or token>
 --- End of inner exception stack trace ---;
webserver!ReportServer_0-1!3464!01/09/2018-14:54:48:: e ERROR: Throwing Microsoft.ReportingServices.Diagnostics.Utilities.UnhandledHttpApplicationException: , Microsoft.ReportingServices.Diagnostics.Utilities.UnhandledHttpApplicationException: The report server encountered an unhandled exception in HttpApplication. ---> System.ArgumentNullException: Value cannot be null.
Parameter name: exception
 at System.Web.UI.AsyncPostBackErrorEventArgs..ctor(Exception exception)
 at System.Web.UI.PageRequestManager.OnPageError(Object sender, EventArgs e)
 at System.EventHandler.Invoke(Object sender, EventArgs e)
 at System.Web.UI.TemplateControl.OnError(EventArgs e)
 at System.Web.UI.Page.HandleError(Exception e)
 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.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)
 --- End of inner exception stack trace ---;