one of the very first checks, is to use an ohm meter and check for "shorts to frame" on all the power cables (must be more than 100k ohms resistance between and power cable and the frame, with the battery disconnected and out, not touching the truck, if you have less resistance between the frame and a cable, trace and fix that before you try further troubleshooting).
Also check the battery for good voltage at rest and under a load (hydraulic stall at full lift is about as good a load as possible). I was suggesting a 'quick and dirty' way to consider if the battery has [with age] gotten to a spot where -it- is a/the 'short to frame'. you would not be checking between battery positive and negative, but between the frame of the battery and one then the other of the power cables.
electric forklifts for the most part (and all ev1 controller equipped trucks), do not use a frame ground, but rather have a completed circuit back to battery negative. both in the control circuits and the power circuits.
is there a backup alarm on this truck? is it correctly wired, both positive and negitive?