Items are sent to translate to SDL TMS from SDL Tridion Sites 8.5/9.1.   The items have comments added to them during the SDL TMS workflow.  The items get markup for these comments denoted with the mrk element.  It is added to the XLIFF and needs to be parsed within Tridion Sites.   But the retrieval fails with the error:
 

unknown tag {urn:oasis:names:tc:xliff:document:1.2}mrk

at Tridion.TranslationManager.DomainModel.TranslationSystems.SdlXliffDocument.ParseMrkElement(XElement element, IList`1 resultTextPartsList)
at Tridion.TranslationManager.DomainModel.TranslationSystems.SdlXliffDocument.ExtractSegments(XElement elementWithSegments, String transUnitId, List`1 xliffSegments)
at Tridion.TranslationManager.DomainModel.TranslationSystems.SdlXliffDocument.ExtractSegmentsFromTranslationUnit(XElement transUnitElement, XName elementName)
at Tridion.TranslationManager.DomainModel.TranslationSystems.SdlXliffDocument.GetTextSegments(Dictionary`2 xliffElementLookup)
at Tridion.TranslationManager.DomainModel.TranslationSystems.SdlXliffDocument.GetTextSegments()
at Tridion.TranslationManager.DomainModel.TranslationSystems.TmsTranslationSystemItem.RetrieveTextSegments()
at Tridion.TranslationManager.DomainModel.Service.RetrieveFromTranslationTask.RetrieveItem(TranslationSystemItem item, TmsTranslationJob tmsJob, IList`1 errorPublicationUris, RetrieveReason retrieveReason)
at Tridion.TranslationManager.DomainModel.Service.RetrieveFromTranslationTask.RetrieveItems(TmsTranslationJob tmsJob, IEnumerable`1 jobStatus, IList`1 errorPublicationUris)

 

A hotfix is available for SDL Tridion Sites 8.5 and SDL Tridion Sites 9.1. 

SDL Tridion Sites 8.5 - TM_8.5.0.17009
SDL Tridion Sites 9.1 - TM_9.1.0.17009

The hotfixes can be obtained via FTP as per:

Where can I find hotfixes for SDL Tridion Sites/Web?

The SDL Tridion Sites XLIFF parser cannot handle comments on items from SDL TMS.