If loose wiring is not the issue try to see that something is not rubbed raw and touching the case somewhere, causing a current drain to ground / chassis. I was having similar issues on my Ribelle with the lights, turns out the one that I replaced, I didn't route the wire right, and it eventually rubbed and would occasionally touch the handlebar metal and throw that fault. Another issue I found was after 'fixing' a dropped bike / light damage, the board in back of it, the connector was damaged and I didn't see it,and that caused the problems until I just replaced the whole light assembly and that fixed it.
Aaron