Error message "Object reference not set to an instance of an object" when working with merged files in review mode

I am using SDL Trados Studio 2017 SR1 - 14.1.10018.54792

When I am working with files merged on the fly in review mode, I get the error message "Object reference not set to an instance of an object" when I try to save them. Also, when I export multiple files from the same project for bilingual review, import the changes and then try to work on the merged files with reviewed changes, I also get the same error message.

This problem only started to occur after I applied an update in November/December 2019. Strangely, according to the release notes, this update was supposed to fix precisely this problem.

Full details of the error message below:

<?xml version="1.0"?>

<SDLErrorDetails time="30/12/2019 15:10:37"><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 Home</OperatingSystem><ServicePack>NULL</ServicePack><OperatingSystemLanguage>2057</OperatingSystemLanguage><CodePage>1252</CodePage><LoggedOnUser>LENOVO-PC\Daisy Waites</LoggedOnUser><DotNetFrameWork>4.0.30319.42000</DotNetFrameWork><ComputerName>LENOVO-PC</ComputerName><ConnectedToNetwork>True</ConnectedToNetwork><PhysicalMemory>8286484 MB</PhysicalMemory></Environment></SDLErrorDetails>