Where can I download right version RouterBoot for RB850G?
I did downgrade to 6.0rc13 beacouse 6.18 is unstable and now.. I don't have access to any ethernet port only via serial port...

6.0rc13? That's soooooo old.
In general you should not downgrade to a lower version than the version the router was shipped with.
I'm pretty sure that 6.0rc13 does not support the RB850G.
Maybe netinstall can fix this.

definitely you should netinstall latest available RouterOS on the router using netinstall tool. You are lucky that router is working at all.