Y-Axis starting point is off about -2.5mm

Hello,

I’m new to cnc and stuff.
Just got this chinese machine 3018 pro last week and everything is going well until this morning.
I updated stock grbl v0.9j.20180122 to v1.1h.20190825 with arduino software and everything went wrong.
COM disconnected for no reason, grblControl not detected (sometimes and i have to restart my pc for it.), starting point is way off (x-axis is off about -40 mm, and y-axis is off about -18 mm).
So i decided to flash it back to stock version, everything is back to normal but y-axis starting point. It’s still off, but only by -2.5 mm.
What should i do to fix this starting point issue?

Fyi,
I did use Candle 1.1 for v1.1h.20190825.

Thanks in advance,
Richard