Glad to see you got the D-750 already, and I’m sorry to hear the N200 and your DAC are not playing together nicely. Do you happen to have a toslink cable? I think maybe one was included with the N200. If you try that and it works, then the SPDIF interfaces “should” be fine and it localized it to the coax connection. If Toslink also drops out, then it is a signal that one of the units’ SPDIF interfaces is problematic as a whole.
If you haven’t already, check it there are firmware upgrades for both units and also try a hard power down on both for a few minutes.
Very odd that the N200 works flawlessly with the RME but not your Lab 12.