Hello,
Here's an update on my problem.
After a few more tests and still at a standstill, I dismantled all the modules from the rack in case one might be causing a problem.
I further reduced the number of modules for testing.
Two configurations were tested:
1) Keystep 37 connected via MIDI to a uDCO with a DIN 5 to mini jack MIDI cable connected to the uDCO's MIDI port, then directly OUT to the mixer.
2) The Nerdseq sending a note or sequence through the Nerdseq's MIDI expander. (MIDI OUT mini jack to MIDI mini jack on the uDCO, then direct OUT on the mixing console)
The problem persisted after removing all the other modules in both setup (1 and 2).
In these tests I noticed that the problem does not come directly from the gate jack which is not even connected. We hear what I suppose to be the Note ON / Note OFF signal in the note pitch which emits the clicks that we hear on the first post
I then moved the setup to another home, with a different electrical circuit. Without success.
I brought it back to the studio.
During my last test session, and to complicate the problem , only one of the three uDCOs exhibited the glitches.
I isolated it and will try again with the other two uDCOs, but I've already had the problem with all three oscillators simultaneously in the past.
I always encounter the same two types of problems.
The good news is that I can eliminate Nerdseq and the midi expender from the list of suspects.
The not so good news is that identifying the problem is complicated; I've spent about ten hours testing and at least as much research.
I think the problem may come from:
I'll try the following:
I received a Bitbox Micro as a Eurorack sampler. Due to MIDI issues I haven't even tried it yet.
I'll try using it via MIDI, controlled by keystep or Nerdseq, and see if I have any problems.
I might ask for help on a more general modular synth forum, although I'm always open to advice if you have any .
In any case, thank you for your quick response. 
Here's an update on my problem.
After a few more tests and still at a standstill, I dismantled all the modules from the rack in case one might be causing a problem.
I further reduced the number of modules for testing.
Two configurations were tested:
1) Keystep 37 connected via MIDI to a uDCO with a DIN 5 to mini jack MIDI cable connected to the uDCO's MIDI port, then directly OUT to the mixer.
2) The Nerdseq sending a note or sequence through the Nerdseq's MIDI expander. (MIDI OUT mini jack to MIDI mini jack on the uDCO, then direct OUT on the mixing console)
The problem persisted after removing all the other modules in both setup (1 and 2).
In these tests I noticed that the problem does not come directly from the gate jack which is not even connected. We hear what I suppose to be the Note ON / Note OFF signal in the note pitch which emits the clicks that we hear on the first post
I then moved the setup to another home, with a different electrical circuit. Without success.
I brought it back to the studio.
During my last test session, and to complicate the problem , only one of the three uDCOs exhibited the glitches.
I isolated it and will try again with the other two uDCOs, but I've already had the problem with all three oscillators simultaneously in the past.
I always encounter the same two types of problems.
- The most common is the clicks that seem to be related to MIDI ON/MIDI OFF triggering. (first post)
- And the second, less frequent glitch, where the note becomes unstable and saturated until it becomes nothing more than noise. I have the impression that it triggers more quickly when I play many notes in a row. Here's a video of the second type of glitch that I hadn't posted.
The good news is that I can eliminate Nerdseq and the midi expender from the list of suspects.
The not so good news is that identifying the problem is complicated; I've spent about ten hours testing and at least as much research.
I think the problem may come from:
- The rack itself, but that seemed strange to me. I have two Arturia 6U racks, and the problem occurs on both.
- I'm researching the DIN5 MIDI cable to mini jack. Some wiring can be problematic. When all five cables are soldered to the DIN connector, some users recommend soldering only the bare minimum for MIDI.
- Or my oscillators, which, for some reason, are incompatible with the rest of my setup.
I'll try the following:
I received a Bitbox Micro as a Eurorack sampler. Due to MIDI issues I haven't even tried it yet.
I'll try using it via MIDI, controlled by keystep or Nerdseq, and see if I have any problems.
I might ask for help on a more general modular synth forum, although I'm always open to advice if you have any .
In any case, thank you for your quick response.