It means you have a short circuit either inside of the VFD, i.e. one of the transistors is shorted, or you have a short circuit in the motor circuit somewhere. You can run a diagnostic of the transistor status from the front keypad, in parameter section 1.10. If they all check out OK, then you have a problem in the wiring to the motor or the motor windings themselves. My money is on the motor windings.
One thing that happens a lot with installing VFDs in old motors, is that the old motor insulation was leaking to ground or phase-to-phase, but the short was of high enough impedance that the Across-the-Line (DOL to you) starter did not trip out because the total current was still below the overload relay trip threshold (often exacerbated by people thinking the OL is bad and "tweaking" it so that is quits tripping).
But the VFD has to be much more careful about things like that and will not tolerate it because it can damage the transistors. Hence the VFD becomes a "fault finder". The problem was already there, you just were not aware of it yet.
"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
� Kilgore Trout (via Kurt Vonnegut)