Can the rubber boot on the RJ45 end of the ProSoft RJ45 to DB9M adapter cable cause communication disruption?

Oddly enough, yes, it can. That rubber boot is meant to protect the plastic clip on the RJ45 connector from being snagged and broken when the cable is pull though tight spaces. However, it can slide back and forth on the cable, as well.

If it slides a little too far forward, toward the RJ45 end of the cable, the cable could be plugged in, the clip could snap into place; but, once released, the boot could cause the RJ45 connector to back out just enough to cause intermittent or total loss of connection and communication.

Fortunately, the solution is simple. Just slide the boot further back down the cable, away from the RJ45 connector.