Most likely this would be the ESC! I have no idea how or even of it can be fixed. I would suppose it could be the timer if it was sending variable frequency as well as variable pulse width output. Try a different ESC and/or a different timer to determine which is faulty.
Wiser heads may explain this - Tim??
I'd like to say that it couldn't be the timer, because any sensibly-designed ESC would refuse to go backwards no matter what. Unfortunately, that leaves open the door for a timer that has nonsense in its design, and after 25 years in industry fixing software screwups (some, alas, my own), all I can do is say it can't be ruled out.
If it didn't do it before, and it does now, there's probably a hardware issue with the ESC. There's a chance that the timer is broken and talking to a design flaw in the ESC. In that case, changing ESC's would result in the same behavior while changing timers would fix it.
I suspect that the backwards running that Paul has seen is just a startup thing -- while I fly slime for CL, I fly mostly electric for RC and I've never seen my, or anyone else's, electric motor run backwards past the first second or so when the ESC is fumbling around trying to figure out how the motor is positioned.