From your description, it seems a motor hall fault but recheck the light combination. Make sure the motor cables is fully pluged in at the chainstay and at the rear rack beside the controller case at end of battery. Any damage to cable where it enters the axle? On my daughters pendleton with the'790' unit, the flashing lights give an indication to the fault. For example, the hi and low lights flashing is a 'brake level' fault and most likely is due to switches on brake levers sticking or brake lever not fully returning to normal position or wiring damage at brake lever. You can disconnect these to see which is faulty and bike will work. There may also be a red light led on the pas sensor behind the large front chainring which turn on with bike but flashes when you pedal.
Other codes on 790 unit for the record are,
Light Combination | Definition |
indicator light flash(LOW level) | abnormal current level |
indicator light flash(MED level) | abnormal throttle level |
indicator light flash(LOW+MED level) | motor phase level |
indicator light flash(HIGH level) | Motor hall signal level |
indicator light flash(LOW+HIGH levels) | brake level |
indicator light flash(LOW+MED+HIGH levels) | Abnormal communication level |
If under guarantee (2 years?), perhaps return to shop.