well my 2 cents after getting message to from administrator to try and help here??
since you stated the issue didn;'t exist when parked,
that tends to make me think, of two things
one a mouse or other critter chewed on some wires some place
OR< like stated, due to now COLDER temps, you got water/moisture in some connection, or over time, corrosion has finally built up enough to cause an issue
as stated by others here, the machine seems to think the seat belt is not connected(when I gather you have it) or the seat safety switch is not being recognized when your sitting on seat
which should lock out some of your controls, like the boom and such
the Fact the machine can over power the parking brake and move the machine, is, due to either, a poorly adjusted parking brake, or worn parks on the parking brake, or this is my guess
SO< that means, IMO< you need to start checking the seat belt and seat witches and all wiring and connections to them,
I would clean all connections and add some dielectric grease to them, to help them STAY good
and again, be looking for any where a mouse has chewed on a wire and is now causing current flow issues, or shorts!
and IMO< (no offense meant to you, ]
joeyt1995 ) many times connections and wire damage can happen without throwing a code, due to they can be erratic on causing the issue and or just the connections being corroded, do not throw codes for corrosion, , countless battery terminals, cause amp flow reduction without throwing codes as an example, or even just grounds not grounding well
so, just cause NO codes, to me, doesn't meant not a wiring issue!