VERIFIED SOLUTION i

MailStream Plus - a new install of MailStream Plus overwrites the previous version's files, such as MSPRun.vbs, which may affect a job completing if it has been customized.

Product affected: MailStream Plus™
Operating System: Windows

Issue

A user had modified the MSPRun.vbs so it would not display a pop-up box at the end.  Instead when it was called by their script, the vbs script would just complete, and their script would continue running.  
But when they installed a new version of MailStream Plus™ (MSP), they were not aware that their people had customized the vbs script in this way, nor that a new installation of MSP would over-write the previous MSPRun.vbs.
So what happened now is that a pop-up window appeared when MSP completed, and the vbs script held the job at that point, and it did not return to their custom script. 

Cause

By default, MSPRun.vbs will normally open a pop-up box when MSP completes, saying that the job is complete, and asking if you want to view the results.

FYI- Start -> All Programs -> Group 1 Software -> MailStream Plus -> Run MailStream Plus Job also calls MSPRun.vbs to run the job.

Resolution

UPDATED: March 13, 2019
The resolution was that they had both a PROD and TEST environment.  When they installed a new version of MSP in TEST, and it stopped working, they were able to copy the modified vbs script from the PROD system, and then the TEST job completed.  
When they install into PROD, they will need to copy the modified vbs script from TEST into PROD.