I have a Toolcat 5600 that can barely drive but does not show error codes. The arm seems to work well even though I have not tried to work with a loaded cube. I raised with the cat and the rear axle seems to move more slowly than the front axle. I read in this forum that disconnecting the speed sensor from the rear axle was a solution for some driving problems. He can be a brake solenoid that is stuck between open / closed? Waiting for it to be a problem of the pump or motor hydraulic.
Most, if not all the Toolcats 5600 of 2003, had problems with the hydraulic drive engines that failed, I think mine lasted around the 700 hour mark. At first you could feel a dead point in the engine where the drive torque fell, and if you had an accessory that uses a large amount of liquid as the cover of the lawn mower or angle broom, you could also feel the accessory of Losing the speed when the engines were at these dead points. If there was little charge on the machine, you may not feel at all, but the maximum speed was lower and falling as it progressed. Going uphill with an accessory in progress was the worst scenario.
If this is your problem, the only solution is to replace both drive engines. You also have to wash the entire hydraulic system and change the two hydraulic filters. You can open the “box drain” filter to find the small pieces of motor that came out. When the engines are self-destructed, they throw small pieces of brass into the system that can obstruct things like the joystick to control the feather.