OK tried that with a new power cycling.
"Make sure sequencer is stopped or Arp is not playing." Check
"Press the record button so it lights up." Check
"Hold a mod step and twiddle the knob." Check
"Does this clear the problem?"
Yes, for as long as Record mode was on. When not in record mode it locks up at +/ 0. Well, that is for the 10 first preset loads or so... eventually it became unstuck and works without engaging Record mode.
Edit: Aaaaand... just as I went back to the synth and did some Record/Play cycles, the Mod Amount locked up again, without a power cycle. Then after a single press of Record it unlocked once more.
Edit 2: Another unrelated thing which makes itself very evident during this testing: when pressing a Mod track button and tweaking Mod Amount in play mode, the Mod step on/off state remains on, and it will even be switched immediately on if held when in the off state and the tweaking the Mod Amount. When in Record mode, the state will always toggle when you tweak Mod Amount while holding the button. The latter seems like a bug, or at least a very annoying "feature". I'd wish the mod track buttons remain in the on state when the Mod Amount is tweaked in all cases. I'll add this to my wishlist thread.