this is a common problem in specialized power systems (was called simpowersystems a while ago). Essentially, inductors are modelled as current sources, and 2 current sources cannot exist in series. The quick fix for this is to place a three phase snubber resistor on the output terminals of the motor. A common snubber could be a wye connected resistor with resistance of 1e6 and grounded. Making this resistor smaller can improve simulation speed, but eventually it will draw enough power to actually impact the quailty of your simulaiton. Making this larger will improve accuracy but decrease simulation speed.
Alternatively, you could switch to using simscape electrical core components (the blue ones) and then you won't get this issue.