The Bobcat S450 had the error code D7554. This code is for the left reverse solenoid. When the code appears I lose reverse gear on the left side. I have moved the valve solenoids to see if the problem follows the solenoid, but it doesn’t. It seems that the problem starts after the machine runs for 15 minutes.
If you have swapped the entire solenoid valves between the sides to try and isolate the problem, but the problem remains the same regardless of the side and you do not follow the solenoid, you are leaning towards a problem, either within the wiring harness or the pins of the connector between the controller and solenoid, or you could have a problem within the controller itself with the output signal going to the solenoid.
I would confirm that the input signal from your controls and wire harness is okay and if so check the outputs by comparing both sides of your travel motors to see if they are holding the same voltage signal to the solenoids.
This will help determine if the problem is the supply voltage going to the solenoid and before the solenoid or if there may be a mechanical problem within that travel motor.
Check the condition of the oil for any signs of fine metal or glitter-like debris, which would indicate a possible component failure or problem.