If, instead of searching for 'MSDTSServer', you search for MS DTS Server, you will have much greater success. 

As to why only one plan failed, it may have to do with the way it was configured initially, the way the duplicate IDs were resolved, or the nature of the plan itself.  It could be that only this plan involved DTS Server, where the conflict arose.  Or only this one plan didn't get the proper mapping - this could have been a network admin's oversight (aka ooopsie) - it was resolved for all but this one before you were involved.

No answer for your last question from me...  Good luck going back and recreating the sequence of events (if you want to?), to determine what happened and what went wrong.  I think it was probably an honest, human error.