1. Login to Trados GroupShare Database server and run the below queries in the same sequence.

select * from sts.Resource where resourceguid = '<Project_GUID_Here>' -- Query 1
select * from proj.ProjectFile where ProjectId = <resourceid_from_Query 1> --Query 2
select * from proj.LanguageFile where ProjectFileId = <Projectfileid_from_Query 2> -- Query 3
(If multiple files present, check files for the TargetLanguage direction in next query else skip next Query.
select * from proj.LanguageDirection where LanguageDirectionId = <LanguageDirectionId_from Query 3>

SELECT F.FileUniqueId,F.LanguageFileId,f.FileTimeStamp,f.VersionNumber,f.LatestVersion,F.Name
FROM   proj.[File] AS F
JOIN proj.LanguageFile AS LF ON F.LanguageFileId = LF.LanguageFileId
JOIN proj.ProjectFile AS PF ON LF.ProjectFileId = PF.ProjectFileId
JOIN sts.[Resource] AS R ON PF.ProjectId = R.resourceId
WHERE  R.resourceGuid like '<Project_GUID_Here>'
 AND LF.LanguageFileUniqueId like '<LanguageFileUniqueID_from_Query 3>' and f.LatestVersion=1 --Query 4

2. Note down the FileUniqueId from Query 4 and ProjectGUID; Login to Trados GroupShare application server.
3. Navigate in Filestore project location(path is available in GroupShare console) with projectGUID; Ex : C:\ProgramData\SDL\SDLServer1\FileStore\<ProjectGUID>
4. Search for the file which is corrupted with FileUniqueId and open file in Notepad++ or any editor.
5. For the specific scenario mentioned in the root-cause, find for text id="" and replace all with id="Qitalic" (In this specific scenario id was missing for italic format, in case if it is found to be missed for other formats, then compare the corrupted file with one previous version of the file available/can be downloaded from Trados Groupshare UI, under list view).
6. Save the file and you should be able to open file in both Trados Studio and Online Editor.

When an editor or a translator is working on a file in Trados GroupShare Online Editor with track changes applied, when the user tries to add or delete track changes in segment data, formatting data is missed while updating the sdlxliff file. 

Since formatting data updates with a blank value, sdlxliff file becomes corrupt and error would be displayed when it is opened in Online Editor or Trados Studio.

For instance, in a scenario, id value for italic was updated with id="" instead of id="Qitalic".