Hi,

Because it works a little bit, it sounds like a noise issue on the signal lines. Usually RS-485 is quite immune to noise.

I used one of these from eBay:
RS-232 to RS-485 Adapter Interface Converter | eBay

I wired it with a shielded twisted pair cable, connecting the shield to earth ONLY at the PC's side.

Can you supply details on how your inverter is wired and connected to the computer?

Cheers,
Jay