The Rena motor/pump housing also has built in thermal protection to avoid motor housing meltdown. If they get enough air in them to cavitate instead of pumping, they will shut down on thermal.
If you check the impeller and all looks well there, check your hose connections (especially the intake side) for air leaks into the system.
Most of time, small air amounts are periodically "belched" out of the return, and it's not really a problem, but it is possible.
I plugged one in after cleaning one time and forgot to watch it for expelling air. Within 2-3 minutes it shut down. The motor housing was "warm" not hot to the touch. Though warm is still not good for plastic the softens with heat.