Battery is a "stupid" component unlike the controller which is "smart". The controller checks the inputs and outputs on start up (measures resistance) and if the resistance is out of range he will declare a component faulty.
I would still concentrate on the motors. Both of them. They are very simple so open them up and blow them.
Check the brushes, if they are shorter then 8 mm you have to replace them.
Take the rotor out of stator and put one side into a drill and the other in the motor housing with the bearing. Spin it (like on a lathe) and clean the commutator with fine sand paper.
Try to connect the motors again.

If you still have an error you should check the lines to the controller and controller itself. It's possible to have an error here of course but that would be the first I heard of. When I had "right/left motor open" error it was always a problem with motors.

Please inform us what did you find when you finish the job.