It turned out that the picomotor controller box #3 (S1107569) was healthy.
We thought this wasn't correctly functioning [1], but it actually works good.
This means that the slow digital system which has been controlling the box is more suspicious at this point.

 We knew that this controller box was not correctly working when it was controlled from the slow digital system (see the details in [1]). This can mean that either/both the box is not functioning or/and the slow digital system is not. Today I removed the box from the top of IOT2L and tested the box itself without the slow digital system by controlling it with the tester board. With this setup I drove a picomotor which was hooked up to the front panel in order to see if the box can drive it with no problems. I tested all eight channels by sequentially connecting the picomotor to each of them. All of them worked without any problems : the drive speed seems correct and both CW and CCW rotation can be performed as designed. Both x and y rotation made sense. The test was done at 500 Hz most of the times. Also I tested channel 1 in a frequency range from 100 Hz to 500 Hz by changing the drive frequency and the rotational speed still seemed (in fact even sounded) healthy to me. I concluded that this box was healthy and put it back on the top of the IOT2L enclosure.

 We will check both hardware and software of the slow digital system at the H1 electronics room.

[1] LHO alog 5938 Comments on " HAM3 ISC IR QPDs are likely broken, in-vac cabling error, IO QPD should be OK"