I’d check the switch at the bottom of the playfield that activates ball eject. Easy to mash contacts together when dropping playfield back into cabinet. This is what happened to me anyways…

You have too many balls in the game

No look at the switch. Mine did exactly this. Here is a pic not of Xenon but another Bally. But switch is the same…9E0C9B06-BEF5-497E-B710-39FF016847C8 (resized).jpeg

I'm not familiar with Bally games of this era but is there a ball trough runaway wire gate under the apron - could that be broken?

This. The game doesn’t know it successfully ejected a ball, so it tries again.

I've had this problem. Screwed with my head for a while. Seems it's a programming glitch...both balls need to be in the outhole after a memory "clear". So picture when it was new out of the box, 2 balls dropped on PF and then turn it on. Not in the trough.

So what has fixed mine more than once was remove the 5101 or cut off the battery charge briefly, reinsert 5101/restore the charge, then before turning on the game drop the 2 balls onto the PF and just let them fall into the outhole. For whatever reason, this "resets" the switch config of the outhole and trough when it loads them, then should work again like it should.