It may mean that you have high inductance motors that won't work in series unless you use 24V power. If that is the case then you should connect both motors in parallel to just one connector. Or, if you are not using the E1 motor output, you can connect one of them to that connector and add this line to config.g, before any M350 or M906 commands:
M584 X0 G1 S2:4 E3
If you can tell us the model number of your Z stepper motors, we can tell you if this is the reason for the problem.
