The job of mono is to run the .exe and .dll files which in this case are portable code, not Windows code.
Application Deployment explains. If you can run mono at all, run mono --version. You need at least 5.
If mono can’t find the .exe, it’s possibly because I forgot to tell you you should either cd or use full path.
It turned out to be quite hard for me (without a Mac) to figure out where things are, but here’s one view. Yours (seen through the ps command, maybe something like ps -auxf | grep mono) may be similar. You can see there how mono is launched, and given a .exe to run. Other .exe files are in the folder too.
I’m also still trying to confirm that you’re going directly to OneDrive, and that your Destination looks like:
I’m still also looking for space history from jobs before the problem rose, to see how it looked back then. Suddenly seeing different results without Duplicati changes would suggest that OneDrive changed then. Possibly this also matches when you changed your size to 100 GB. I don’t know why 999995129856 is still being reported by OneDrive, and I doubt Duplicati made it up – it’s an odd value. How far back is it?