Running a CopyTrans and a TM Merge Translations at the same time causes a collision

1. go to a project with a lot of translations, e.g. Zanata Server / master
2. Select TM Merge Translations from the menu
3. Select 'from all' and start the merge, close the dialog
4. Select CopyTrans from the menu
5. Select Continue for all and start the copytrans

Shortly, the CopyTrans halts and this eg. shows in the logs

06:20:39,734 ERROR [org.zanata.service.impl.TranslationMemoryServiceImpl] (pool-11-thread-1) Unexpected TextFlowTarget (state NeedReview): HTextFlowTarget{contents=[Une erreur inattendue s?est produite. Veuillez signaler ce probl?me en d?taillant ce que vous tentiez d?effectuer.], locale=HLocale(localeId=fr), state=NeedReview, comment=HSimpleComment(auto translated by TM merge from project: Zanata Server, version: release, DocId: zanata-war/src/main/resources/messages), textFlow=[An unexpected error has occurred. Please report this problem with details of what you were attempting.]}. You may need to re-index.