it is unlikely that it is the controller; when such errors were reported, it was always a cable or connector issue. Sometimes a multimeter doesn’t help as the error may be present in one moment and have disappeared in the other. It seems you have wrong signals on the stepper, so I would check the cables and connectors by replacing or exchanging them.
It could also be a EMI issue because the Onefinity stepper cables are not shielded. Heavy sources of EMI are the router’s carbon brushes (over the air, by design), or a not properly shielded, grounded and bonded spindle/VFD (or machine). See spindle cable and spindle cable). But I would think of the stepper motor cables or connectors first.