Please switch the GUI to English and then post the full text of the error message, which you can access when clicking "show details"

Thanks, GUI is actually in English, the error message comes up in Windows' region. Full text is exactly the same (see below), I put the translation in the subject line for this message.

Thank you. Please click the disk symbol, save the error details and post them here. This can help RWS people to help you. In the meantime you could also click the link to knowledge base, if not yet done.

Thanks and sorry for the misunderstanding. I am pasting the xml below. Just a general note, I experience the same issue (and used to experience with 2019 sporadically) with different error messages at times.

<SDLErrorDetails time="1.10.2021 20:36:45">
  <ErrorMessage>Değer null olamaz.
Parametre adı: openXmlPart</ErrorMessage>
  <Exception>
    <Type>System.ArgumentNullException, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</Type>
    <ParamName>openXmlPart</ParamName>
    <HelpLink />
    <Source>DocumentFormat.OpenXml</Source>
    <HResult>-2147467261</HResult>
    <StackTrace><![CDATA[   konum: DocumentFormat.OpenXml.OpenXmlPartReader..ctor(OpenXmlPart openXmlPart)
   konum: Sdl.FileTypeSupport.Filters.BilingualExcel.Services.ContentExtractors.ThemeExtractor.ExtractTheme(ThemePart themeDefinitionsPart)
   konum: Sdl.FileTypeSupport.Filters.BilingualExcel.Writer.BilingualExcelWriter.OpenSpreadsheetDocumentAndInitializeContext()
   konum: Sdl.FileTypeSupport.Filters.BilingualExcel.Writer.BilingualExcelWriter.PrepareAndInitializeTargetFile()
   konum: Sdl.FileTypeSupport.Filters.BilingualExcel.Writer.BilingualExcelWriter.SetOutputProperties(INativeOutputFileProperties properties)
   konum: Sdl.FileTypeSupport.Framework.Integration.FileGenerator.CallSetOutputProperties()
   konum: Sdl.FileTypeSupport.Framework.Integration.FileGenerator.SetFileProperties(IFileProperties fileInfo)
   konum: Sdl.FileTypeSupport.Framework.BilingualApi.AbstractBilingualContentProcessor.SetFileProperties(IFileProperties fileInfo)
   konum: Sdl.FileTypeSupport.Framework.Integration.AbstractBilingualProcessorContainer.SetFileProperties(IFileProperties fileInfo)
   konum: Sdl.FileTypeSupport.Framework.Integration.MultiFileConverter.SetFileProperties(IFileProperties fileInfo)
   konum: Sdl.FileTypeSupport.Framework.BilingualApi.AbstractBilingualContentProcessor.SetFileProperties(IFileProperties fileInfo)
   konum: Sdl.FileTypeSupport.Framework.Integration.AbstractBilingualProcessorContainer.SetFileProperties(IFileProperties fileInfo)
   konum: Sdl.FileTypeSupport.Bilingual.SdlXliff.XliffFileReader.OnEndFileHeader()
   konum: Sdl.FileTypeSupport.Bilingual.SdlXliff.SdlXliffFeeder.<>c.<HandleFileHeader>b__16_1(ISdlXliffStreamContentHandler handler)
   konum: System.Collections.Generic.List`1.ForEach(Action`1 action)
   konum: Sdl.FileTypeSupport.Bilingual.SdlXliff.SdlXliffFeeder.HandleFileHeader()
   konum: Sdl.FileTypeSupport.Bilingual.SdlXliff.SdlXliffFeeder.ContinueScanning()
   konum: Sdl.FileTypeSupport.Bilingual.SdlXliff.XliffFileReader.ContinueParsing()
   konum: Sdl.FileTypeSupport.Bilingual.SdlXliff.XliffFileReader.ParseNext()
   konum: Sdl.FileTypeSupport.Framework.Integration.FileExtractor.ParseNext()
   konum: Sdl.FileTypeSupport.Framework.Integration.MultiFileConverter.ParseNext()
   konum: Sdl.FileTypeSupport.Framework.Integration.MultiFileConverter.Parse()
   konum: Sdl.ProjectApi.AutomaticTasks.NativeFileGenerator.GenerateFiles(ContentRestriction restriction, Boolean preview, Int32 filePercentBase, Int32 filePercentFactor)
   konum: Sdl.ProjectApi.AutomaticTasks.GenerateTargetTranslation.GenerateTargetTranslationTask.ProcessFile(IExecutingTaskFile executingTaskFile)
   konum: Sdl.ProjectApi.AutomaticTasks.AbstractFileLevelAutomaticTaskImplementation.Execute()]]></StackTrace>
  </Exception>
  <Environment>
    <ProductName>SDL Trados Studio</ProductName>
    <ProductVersion>Studio16</ProductVersion>
    <EntryAssemblyFileVersion>16.1.8.4404</EntryAssemblyFileVersion>
    <OperatingSystem>Microsoft Windows 10 Home Single Language</OperatingSystem>
    <ServicePack>NULL</ServicePack>
    <OperatingSystemLanguage>1055</OperatingSystemLanguage>
    <CodePage>1254</CodePage>
    <LoggedOnUser>DESKTOP-UTC16GV\Lenovo</LoggedOnUser>
    <DotNetFrameWork>4.0.30319.42000</DotNetFrameWork>
    <ComputerName>DESKTOP-UTC16GV</ComputerName>
    <ConnectedToNetwork>True</ConnectedToNetwork>
    <PhysicalMemory>16647476 MB</PhysicalMemory>
  </Environment>
</SDLErrorDetails>

Thanks and sorry for the misunderstanding. I just posted the details but my post got flagged, it'll probably take a while.

Yes, that happens, if you copy & paste xml in the message body. Please zip the xml file instead and attach it to the post, that should go trough immediately.

sdlerror-2021101-20h36m35s.sdlerror.zip

My knowledge is not enough to answer this question. Have you translated the worksheet name? If so, maybe the culprit is there.

Otherwise I hope or Paul can help.

I think your knowledge may be enough

The first job today had German sheet names. The second one I created a new file because the original one was would be too complicated for Studio - so Turkish sheet names.

I will play with the second file and confirm.

Changed sheet name to English and created new project. Problem still persists.

Thanks for all your help