This is a very convoluted setup, but effectively this is what goes down: Manager service (which I don't have control over) running as admin user X starts my executable, which then starts Java as user Y using the standard c# StartInfo.Username/Password controls.

Now, from a basic (not elevated or anything, just admin) command prompt I can run that executable, and Java pops up and works fine, running perfectly under the user it should be. When the service runs the same executable, however, Java silently fails. The only hint I see is this series of events in the event viewer:

• Service starts
• "Application popup: java.exe - Application Error : The application was unable to start correctly (0xc0000142). Click OK to close the application. " (googling this reveals a lot of scam sites telling me to use their "free antivirus to fix 0xc0000142 errors easy!"... sigh)
• Service stops (the java shutdown propagated, which is supposed to happen)

And here's what process explorer has for the failure:

As you can see, everything shows as a success.

Now, I think this might have something to do with the permissions (the user java.exe is running under has traverse permission for the entire drive and full permissions to Directory A, which is where the .jar is), but I just can't fathom how something that works fine from the command line (and, this is an upgrade, the previous system without the user-switching aspect works fine from the service) can fail with such a cryptic message and little showing up in logs.