VESC 6 PLUS should only run with original VESC-Tool FW.
Best thing to do: Re-Program it via SWD Prog. Use the working VESC 6 to flash the non working VESC.
Interconnect both SWD ports (GND, IO, CLK only), cables not longer than 8cm. Power both units, open SWD Prog tab in VESC-Tool, hit connect, choose VESC 6, hit upload.