SharePoint 2016: three-state approval workflows suddenly don't work anymore

We are currently encountering a strange behaviour with approval workflow in SharePoint 2016 on-prem.

We are using a three-state approval workflow on a specific library. Our current version of SharePoint 2016 is 16.0.5056.1001 and September CU was deployed a few days ago.

For an unknown reason, some approval workflows throw the following error:

Warning: string "Exception occurs while approving file '{0}'. Details: {1}" could not be formatted properly, as it raised the following exception: System.FormatException: Index (zero based) must be greater than or equal to zero and less than the size of the argument list.    
at System.Text.StringBuilder.AppendFormatHelper(IFormatProvider provider, String format, ParamsArray args)    
at System.String.FormatHelper(IFormatProvider provider, String format, ParamsArray args)    
at System.String.Format(IFormatProvider provider, String format, Object[] args)    
at Microsoft.SharePoint.Administration.SPDiagnosticsServiceBase.FormatStringForLog(String unformatted, Object[] data) StackTrace:  
at Microsoft.SharePoint.Administration.SPDiagnosticsServiceBase.WriteTrace(UInt32 id, SPDiagnosticsCategory category, TraceSeverity severity, String output, Object[] data)     
at System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter)    
at Microsoft.SharePoint.WebPartPages.SPChrome.RenderPartContents(HtmlTextWriter output, WebPart part)    
at Microsoft.SharePoint.WebPartPages.SPChrome.RenderWebPart(HtmlTextWriter output, WebPart part)    
at Microsoft.SharePoint.WebPartPages.WebPartZone.RenderZoneCell(HtmlTextWriter output, Boolean bMoreParts, WebPart part)    
at Microsoft.SharePoint.WebPartPages.WebPartZone.RenderWebParts(HtmlTextWriter output, ArrayList webParts)    
at Microsoft.SharePoint.WebPartPages.WebPartZone.Render(HtmlTextWriter output)    
at System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter)    
at System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children)    
at System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter)    
at System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children)    
at System.Web.UI.HtmlControls.HtmlContainerControl.Render(HtmlTextWriter writer)    
at System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter)    
at System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children)    
at System.Web.UI.HtmlControls.HtmlForm.RenderChildren(HtmlTextWriter writer)    
at System.Web.UI.HtmlControls.HtmlContainerControl.Render(HtmlTextWriter writer)    
at Microsoft.SharePoint.WebControls.SharePointForm.Render(HtmlTextWriter output)    
at System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter)    
at System.Web.UI.HtmlControls.HtmlForm.RenderControl(HtmlTextWriter writer)      
at System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children)    
at System.Web.UI.HtmlControls.HtmlContainerControl.Render(HtmlTextWriter writer)    
at System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter)    
at System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children)    
at System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter)    
at System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children)    
at Microsoft.SharePoint.Publishing.PublishingCachablePage.RenderChildren(HtmlTextWriter writer)    
at System.Web.UI.Page.Render(HtmlTextWriter writer)    
at Microsoft.SharePoint.WebControls.DeltaPage.RenderToBase(HtmlTextWriter writer)    
at Microsoft.SharePoint.WebControls.DeltaPage.Render(HtmlTextWriter writer)    
at Microsoft.SharePoint.WebPartPages.WebPartPage.Render(HtmlTextWriter writer)    
at System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter)    
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 Microsoft.SharePoint.Publishing.TemplateRedirectionPage.ProcessRequest(HttpContext context)    
at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()    
at System.Web.HttpApplication.ExecuteStepImpl(IExecutionStep step)    
at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)    
at System.Web.HttpApplication.PipelineStepManager.ResumeSteps(Exception error)    
at System.Web.HttpApplication.BeginProcessRequestNotification(HttpContext context, AsyncCallback cb)    
at System.Web.HttpRuntime.ProcessRequestNotificationPrivate(IIS7WorkerRequest wr, HttpContext context)    
at System.Web.Hosting.PipelineRuntime.ProcessRequestNotificationHelper(IntPtr rootedObjectsPointer, IntPtr nativeRequestContext, IntPtr moduleData, Int32 flags)    
at System.Web.Hosting.PipelineRuntime.ProcessRequestNotification(IntPtr rootedObjectsPointer, IntPtr nativeRequestContext, IntPtr moduleData, Int32 flags)    
at System.Web.Hosting.UnsafeIISMethods.MgdIndicateCompletion(IntPtr pHandler, RequestNotificationStatus& notificationStatus)    
at System.Web.Hosting.UnsafeIISMethods.MgdIndicateCompletion(IntPtr pHandler, RequestNotificationStatus& notificationStatus)    
at System.Web.Hosting.PipelineRuntime.ProcessRequestNotificationHelper(IntPtr rootedObjectsPointer, IntPtr nativeRequestContext, IntPtr moduleData, Int32 flags)    
at System.Web.Hosting.PipelineRuntime.ProcessRequestNotification(IntPtr rootedObjectsPointer, IntPtr nativeRequestContext, IntPtr moduleData, Int32 flags)

Unfortunately, we cannot figure out what is happening. The only other information about this error is the following message:

Exception occurs while approving file '{0}'. Details: {1}

We don’t see anything more in the logs or through the server’s events. This problem came out of nowhere because everything worked fine before and after the last update. It also still works in our test environment.

We also tried to run iisreset and to restart the SharePoint Timer Service for each server of the farm.

We tried to delete and recreate the workflows, checked the permissions, unchecked the "update the approval status" option, remove approval on the concerned library, but without any success. The library was not changed recently.

The problem related to the .NET update in September 2018 was also fixed a long time ago.

However, we can still approve manually and by-pass the workflow.

Has anyone encountered such a behaviour? Any help would be much appreciated.

Many thanks in advance.

Best regards,