Cannot save file. Warning: Value cannot be null. Parameter name: input

I have a huge document which I cannot save. If I try saving through Advanced Save - Save source I get "Failed to save source content: Illegal characters in path" If I try to export as Bilingual I get " Value cannot be null. Parameter name: input" Furthermore most of the functions in the ribbon are greyed out! This is a reviewed document and if I have to start afresh and batch tasks - pre translate, all the changes will be gone. Any help please. I have Studio 2021

<?xml version="1.0"?>

-<SDLErrorDetails time="04/05/2021 17:23:13">

<ErrorMessage>Value cannot be null. Parameter name: input</ErrorMessage>


-<Exception>

<Type>System.ArgumentNullException, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</Type>

<ParamName>input</ParamName>

<HelpLink/>

<Source>System</Source>

<HResult>-2147467261</HResult>


-<StackTrace>

-<![CDATA[ at System.Text.RegularExpressions.Regex.Replace(String input, String replacement)
at System.Text.RegularExpressions.Regex.Replace(String input, String pattern, String replacement)
at Sdl.FileTypeSupport.Framework.Core.Utilities.BilingualApi.InvalidEncodingRemovalVisitor.VisitCommentMarker(ICommentMarker commentMarker)
at Sdl.FileTypeSupport.Framework.Bilingual.CommentMarker.AcceptVisitor(IMarkupDataVisitor visitor)
at Sdl.FileTypeSupport.Framework.Core.Utilities.BilingualApi.InvalidEncodingRemovalVisitor.VisitChildren(IAbstractMarkupDataContainer container)
at Sdl.FileTypeSupport.Framework.Core.Utilities.BilingualApi.InvalidEncodingRemovalVisitor.VisitSegment(ISegment segment)
at Sdl.FileTypeSupport.Framework.Bilingual.Segment.AcceptVisitor(IMarkupDataVisitor visitor)
at Sdl.FileTypeSupport.Framework.Core.Utilities.BilingualApi.InvalidEncodingRemovalVisitor.VisitChildren(IAbstractMarkupDataContainer container)
at Sdl.FileTypeSupport.Framework.Core.Utilities.BilingualApi.InvalidEncodingRemovalVisitor.VisitTagPair(ITagPair tagPair)
at Sdl.FileTypeSupport.Framework.Bilingual.TagPair.AcceptVisitor(IMarkupDataVisitor visitor)
at Sdl.FileTypeSupport.Framework.Core.Utilities.BilingualApi.InvalidEncodingRemovalVisitor.VisitChildren(IAbstractMarkupDataContainer container)
at Sdl.FileTypeSupport.Framework.Core.Utilities.BilingualApi.InvalidEncodingRemovalVisitor.ProcessParagraphUnit(IParagraphUnit pu)
at Sdl.FileTypeSupport.Framework.Integration.FileExtractor.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.SaveBilingualImpl(String[] bilingualFilePaths, Boolean copy, IJobExecutionContext context, Boolean isAutoSaved)
at Sdl.TranslationStudio.Editor.TranslationEditor.TranslatableDocument.Save(IJobExecutionContext context)
at Sdl.TranslationStudio.Api.Editor.SaveJobRequest.Execute(IJobExecutionContext context)
at Sdl.Desktop.Platform.Implementation.Services.Job.<_worker_DoWork>b__47_0()
at Sdl.Desktop.Logger.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>Studio16</ProductVersion>

<EntryAssemblyFileVersion>16.1.4.4184</EntryAssemblyFileVersion>

<OperatingSystem>Microsoft Windows 10 Home</OperatingSystem>

</SDLErrorDetails>