Uninstalling / reinstalling your C++ runtimes and / or Queue Manager on those machines, may be worthwhile as troubleshooting?

The other thing to consider is if there is anything else on these machines... like a anti-virus program, running in on-demand mode, or with "heuristic" or "suspicious behaviour" detection enabled... you never know... something like that might be causing an issue?

I should couch this advice by saying that I've not had this precise issue and I'm personally running Queue Manager on OS X... so I'm just making some troubleshooting suggestions here...

It does sound like its probably worth you submitting a support ticket request to Smith Micro?

https://support.smithmicro.com/

EDIT: cross-posted with aRtBee... who knows much more about this stuff! Cheers