If you measure right at the transformer, feeding it 480 v and getting 204 v, you could be one tap low and raising it 2.5% would put you at 209 v. If the primary side 480 v is low, the low voltage problem is upstream, not at the transformer taps. If the unit is rated 208 and you are feeding it 204v solid under load, 204v is not low considering voltage drop downstream, loaded. Not great but not low.
If the unit draws 17 amps L to L at 208v and 17 amps on the neutral, the high neutral current indicates high harmonic content of the load. Today's IT equipment generally has high quality, low harmonic, power supplies. I usually only see cheap power supplies now on the display panels, if your load was all monitor screens. Modern hardware generally is pretty good.
Try to measure neutral current relative to the expected neutral imbalance current, both line and load side of the UPS, to try to determne if the high harmonic content load is after the UPS or is the UPS itself internally.
Most electronic stuff, UPS's included, have electrolytic type capacitors that dry out and can have a service lifetime in a UPS of 5 to 8 years. If the caps are out of spec and distorting the input or output waveform, it could easily be a problem internal to the UPS. I would assume factory service necessary to diagnose and change something like that. Never heard of an "onsite UPS tech".
The UPS display may or may not give you more information, load balance, power factor of the load, reason for going to battery. If the power factor is bad, I would again suspect an internal UPS problem. The electrolytic caps are a wear item.
If you have bad power factor and high harmonic content due to the UPS, it indicates a problem internal to the UPS. If it is caused by the load on the UPS, the UPS could be suffering from it.
As noted above, the UPS's require annual or so maintenance.