Y-Axis motors behave weird after some time of non-use

[t0bias]

Hi,

I have not used my printer for some time (since we have had our first child) und now the Y-Axis behave very strangely; one of the two motors seems to have the belt quit violently slip one tooth every like 80mm or so of movement.
It's twice the exact same stepper, same stepper motor driver moving of the same signal, same length of gt2-belt, same tensioner (I carefully counted revoloutions to make sure, they are equally tensioned).
The Y-Axis with the whole printhead-assembly can move freely with almost no resistance at all when the motors are not engaged.
To be frank, I have no clue how this is even possible, or where to look.
I marked both gears with a Sharpie and compared the rotation of the two motors, and I think it's not the motors that are moving unequally.
Any suggestions on how to narrow down the cause of this issue?

[Phaedrux]

If you remove the belts and try turning the motor with your fingers does it turn smoothly over the full rotation?

Photos of the printer?

After a time of long unuse I might think of lubrication?