Hi

What is your earth resistance test from the earth bar in the minisub to ground - You should get a reading of below 1 Ohm .

If your earth and neutral is bridge in the minisub and your earth to ground reading is acceptable you should be OK.

As a test you could bridge the neutral and earth at the input to the UPS and see if that settles it - If not then they are missing something on the UPS side.

What is actually happening - Is the UPS not starting at all , or going into standby when they operate the machine.
Why are they saying that the neutral is floating