When trying to save target, I get this error message:

Failed to save target content: Value cannot be null. Parameter name: source

There is no additional information (like the "Dependency file not found" issue) and the parameter name is not "file name" like other posts on this error. 

I have not encountered this issue before in about 15 years of using Trados. 

Of course I am working against a deadline so a quick solution would be very welcome. :-)

Here's the error dump:

<SDLErrorDetails time="21.01.2021 14:13:20">
<ErrorMessage>Failed to save target content: Value cannot be null.
Parameter name: source</ErrorMessage>
<Exception>
<Type>System.ArgumentNullException, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</Type>
<ParamName>source</ParamName>
<HelpLink />
<Source>Sdl.FileTypeSupport.Filters.Word</Source>
<HResult>-2147467261</HResult>
<StackTrace><![CDATA[ at Sdl.FileTypeSupport.Filters.Word.Common.FilterException..ctor(Exception ex)
at Sdl.FileTypeSupport.Filters.Word.FileFormat.DocumentControllerFramework.AbstractTransformer.TransformExistingDocument(RuleFileType ruleFileType)
at Sdl.FileTypeSupport.Filters.Word.FrameworkImpl.Generator.WriterImpl.BuildParagraph(String xmlString)
at Sdl.FileTypeSupport.Filters.Word.FrameworkImpl.Generator.AbstractEventsBuilder.ChangeContext(IContextProperties contexts)
at Sdl.FileTypeSupport.Filters.Word.FrameworkImpl.Generator.Specialized.EventsBuilder.ChangeContext(IContextProperties contexts)
at Sdl.FileTypeSupport.Filters.Word.FrameworkImpl.Generator.FileWriter.ChangeContext(IContextProperties contexts)
at Sdl.FileTypeSupport.Framework.NativeApi.AbstractNativeExtractionGenerationContentProcessor.ChangeContext(IContextProperties contexts)
at Sdl.FileTypeSupport.Framework.Integration.NativeWriterMessagesProxy.ChangeContext(IContextProperties newContexts)
at Sdl.FileTypeSupport.Framework.Core.Utilities.NativeApi.Buffer.NativeBuffer.ChangeContext(IContextProperties contexts)
at Sdl.FileTypeSupport.Filters.Processors.WhitespaceBetweenSegmentsProcessor.ChangeContext(IContextProperties newContexts)
at Sdl.FileTypeSupport.Framework.Core.Utilities.NativeApi.Buffer.NativeBuffer.ChangeContext(IContextProperties contexts)
at Sdl.FileTypeSupport.Filters.Processors.FeedbackRemovalProcessor.ChangeContext(IContextProperties newContexts)
at Sdl.FileTypeSupport.Framework.NativeApi.AbstractNativeGenerationContentProcessor.ChangeContext(IContextProperties contexts)
at Sdl.FileTypeSupport.Framework.Core.Utilities.NativeApi.Buffer.ChangeContextContentItem.Invoke(IAbstractNativeContentHandler output)
at Sdl.FileTypeSupport.Framework.Core.Utilities.NativeApi.Buffer.NativeBuffer.Release()
at Sdl.FileTypeSupport.Framework.Native.CommentMerger.ParagraphUnitEnd()
at Sdl.FileTypeSupport.Framework.Core.Utilities.NativeApi.Buffer.ParagraphUnitEndContentItem.Invoke(IAbstractNativeContentHandler output)
at Sdl.FileTypeSupport.Framework.Core.Utilities.NativeApi.Buffer.NativeBuffer.Release()
at Sdl.FileTypeSupport.Framework.Native.AdjacentClonedPairedTagsMerger.StopHolding()
at Sdl.FileTypeSupport.Framework.Native.AdjacentClonedPairedTagsMerger.InlineStartTag(IStartTagProperties tagInfo)
at Sdl.FileTypeSupport.Framework.Core.Utilities.NativeApi.Buffer.NativeBuffer.InlineStartTag(IStartTagProperties tagInfo)
at Sdl.FileTypeSupport.Framework.Bilingual.BilingualToSourceConverter.VisitTagPair(ITagPair tag)
at Sdl.FileTypeSupport.Framework.Bilingual.TagPair.AcceptVisitor(IMarkupDataVisitor visitor)
at Sdl.FileTypeSupport.Framework.Bilingual.BilingualToSourceConverter.VisitContainerItems(IEnumerable`1 container)
at Sdl.FileTypeSupport.Framework.Bilingual.BilingualToSourceConverter.ProcessParagraphUnitContent(IParagraphUnit paragraphUnit)
at Sdl.FileTypeSupport.Framework.Bilingual.BilingualToSourceConverter.ProcessParagraphUnit(IParagraphUnit paragraphUnit)
at Sdl.FileTypeSupport.Framework.BilingualApi.AbstractBilingualContentProcessor.ProcessParagraphUnit(IParagraphUnit paragraphUnit)
at Sdl.FileTypeSupport.Filters.Processors.SubContentEntityForwardConversionBilingualProcessor.ProcessParagraphUnit(IParagraphUnit paragraphUnit)
at Sdl.FileTypeSupport.Framework.BilingualApi.AbstractBilingualContentProcessor.ProcessParagraphUnit(IParagraphUnit paragraphUnit)
at Sdl.FileTypeSupport.Filters.Word.FrameworkImpl.Generator.Specialized.WordBilingualProcessor.ProcessParagraphUnit(IParagraphUnit paragraphUnit)
at Sdl.FileTypeSupport.Framework.BilingualApi.AbstractBilingualContentProcessor.ProcessParagraphUnit(IParagraphUnit paragraphUnit)
at Sdl.FileTypeSupport.Framework.Integration.GenerationBilingualContentLocator.ProcessParagraphUnit(IParagraphUnit paragraphUnit)
at Sdl.FileTypeSupport.Framework.Integration.AbstractBilingualProcessorContainer.ProcessParagraphUnit(IParagraphUnit paragraphUnit)
at Sdl.FileTypeSupport.Framework.Integration.FileGenerator.ProcessParagraphUnit(IParagraphUnit paragraphUnit)
at Sdl.FileTypeSupport.Framework.Integration.AbstractBilingualProcessorContainer.ProcessParagraphUnit(IParagraphUnit paragraphUnit)
at Sdl.FileTypeSupport.Framework.BilingualApi.AbstractBilingualContentProcessor.ProcessParagraphUnit(IParagraphUnit paragraphUnit)
at Sdl.FileTypeSupport.Framework.Integration.LocationMarkerLocator.ProcessParagraphUnit(IParagraphUnit paragraphUnit)
at Sdl.FileTypeSupport.Framework.Integration.AbstractBilingualProcessorContainer.ProcessParagraphUnit(IParagraphUnit paragraphUnit)
at Sdl.FileTypeSupport.Bilingual.Tmfc.TmfcReaderImpl.vv_Visit(TmfcReaderImpl* , FrameworkParagraphContainerField* field)
at FrameworkParagraphContainerField.vv_AcceptFrameworkVisitor(FrameworkParagraphContainerField* , FrameworkFieldVisitor* visitor)
at Sdl.FileTypeSupport.Bilingual.Tmfc.TmfcReaderImpl.vv_VisitStartEndField(TmfcReaderImpl* , StartEndField* startEnd)
at Sdl.FileTypeSupport.Bilingual.Tmfc.TmfcReaderImpl.b_VisitNext(TmfcReaderImpl* )
at Sdl.FileTypeSupport.Bilingual.Tmfc.TmfcReader.ParseNext()
at Sdl.FileTypeSupport.Framework.Integration.FileExtractor.ParseNext()
at Sdl.FileTypeSupport.Framework.Integration.MultiFileConverter.ParseNext()
at Sdl.FileTypeSupport.Framework.Integration.MultiFileConverter.Parse()
at Sdl.TranslationStudio.Editor.TranslationEditor.TranslatableDocument.SaveMonolingualAsJobRequest.Execute(IJobExecutionContext context)
at Sdl.Desktop.Platform.Implementation.Services.Job.<_worker_DoWork>b__3()
at Sdl.Desktop.Platform.Implementation.Services.Log.Resources(Object message, Action action)
at Sdl.Desktop.Platform.Implementation.Services.Job._worker_DoWork(Object sender, DoWorkEventArgs e)
at System.ComponentModel.BackgroundWorker.OnDoWork(DoWorkEventArgs e)
at System.ComponentModel.BackgroundWorker.WorkerThreadStart(Object argument)]]></StackTrace>
</Exception>
<Environment>
<ProductName>SDL Trados Studio</ProductName>
<ProductVersion>11.0.0.0</ProductVersion>
<EntryAssemblyFileVersion>11.2.4435.12</EntryAssemblyFileVersion>
<OperatingSystem>Microsoft Windows 10 Home</OperatingSystem>
<ServicePack>NULL</ServicePack>
<OperatingSystemLanguage>1033</OperatingSystemLanguage>
<CodePage>1252</CodePage>
<LoggedOnUser>Glen\Glen</LoggedOnUser>
<DotNetFrameWork>4.0.30319.42000</DotNetFrameWork>
<ComputerName>GLEN</ComputerName>
<ConnectedToNetwork>True</ConnectedToNetwork>
<PhysicalMemory>6209680 MB</PhysicalMemory>
</Environment>
</SDLErrorDetails>