If you had 10v and after plugging in it dropped to 1.37v something is going wrong, sounds like something is shorted. Based on that I wouldn't connect another 10v source with any current behind it until you figure out why it drops.Update, made another cable with a different trs plug and same response, no response. For funs and grins I inserted it into the output jack and took measurements, on one of the ends, can't remember if it was tip or ring it read 1.37v so I thought that was interesting. It does not read anything coming from the input.
I guess the next step would be to open it up, kind of reluctant to do that but not sure how else to troubleshoot it. Not real sure with doing that what I am going to be looking for, it could be that the mars aqua analaog output is just not what it wants to see, I do not have a Kessil controller on hand to verify, if I did I wouldn't need this.
The Kessil uses the same 0-10v as the Nicrew, it sounds like the converter is doing it's thing as you can control the output signal, not much can go wrong with them, I use the same opamp and never had a problem with them. If you want to stay on the safe side I would suggest getting a TRRS plug or cable as someone did mention they smoked a Pi with a TRS but it worked fine with a TRRS. When you have the TRRS plugged in light you can verify the wire that has 24v so you don't use it. If that doesn't work then you know something is wrong with light or converter. Right now it could be a cable issue.