Object reference not set to an instance of an object

Please help me!!

Software: SDL Trados Studio 2017 SR1 - 14.1.10018.54792.

Computer: Windows10, 64bit, Intel i5

Issue;

- This happens often from second half of last year.

- When I select all files by Ctrl+A and then open it, it shows the error message of "Object reference not set to an instance of an object" after clicking Ctrl+S (saving). So I cannot save the files.

- If I open one file, there is no such error. So now I open it one by one, but it is very ineffective.

To make sure what is problem, I bought a new computer and I installed trados 2017 only and shows same error message.

And then I also installed Trados 2021 and found no such error message with the same files.

And I also installed Trados 2015 and found no such error message with the same files.

I have tried many solutions according to advices from SDL community. But all failed.

<SDLErrorDetails time="4/21/2021 12:12:56 AM">
  <ErrorMessage>Object reference not set to an instance of an object.</ErrorMessage>
  <Exception>
    <Type>System.NullReferenceException, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</Type>
    <HelpLink />
    <Source>Sdl.TranslationStudio.Api</Source>
    <HResult>-2147467261</HResult>
    <StackTrace><![CDATA[   at Sdl.TranslationStudio.Api.Common.SdlxliffXmlDocumentHelper.RemoveUnwantedCommentDefinitions(XDocument childDocument, XmlNamespaceManager nsManager)
   at Sdl.TranslationStudio.Api.Common.SdlxliffXmlDocumentHelper.TransferDocumentInfo(XDocument mergedDocument, XDocument childDocument, XmlNamespaceManager nsManager)
   at Sdl.TranslationStudio.Api.Common.MergedFileChildrenPersister.CreateChildFilesDocuments(XmlNamespaceManager nsManager)
   at Sdl.TranslationStudio.Api.Common.MergedFileChildrenPersister.SaveChildFiles(String outputFile, String[] bilingualFilePaths)
   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.AbstractInternalDocument.SaveJobRequest.Execute(IJobExecutionContext context)
   at Sdl.Desktop.Platform.Implementation.Services.Job.<_worker_DoWork>b__46_0()
   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>14.0.0.0</ProductVersion>
    <EntryAssemblyFileVersion>14.1.10018.54792</EntryAssemblyFileVersion>
    <OperatingSystem>Microsoft Windows 10 Pro</OperatingSystem>
    <ServicePack>NULL</ServicePack>
    <OperatingSystemLanguage>1033</OperatingSystemLanguage>
    <CodePage>949</CodePage>
    <LoggedOnUser>DESKTOP-OU7QUEU\choky</LoggedOnUser>
    <DotNetFrameWork>4.0.30319.42000</DotNetFrameWork>
    <ComputerName>DESKTOP-OU7QUEU</ComputerName>
    <ConnectedToNetwork>True</ConnectedToNetwork>
    <PhysicalMemory>16688572 MB</PhysicalMemory>
  </Environment>
</SDLErrorDetails>

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