I don't want the players to be hurt when they are regenerating so I wrote the script.
And the error just occurred when I was attacked.

For the CrT's end: CraftTweaker/CraftTweaker#1590

Minecraft 1.16.5
Forge 36.2.39
Regeneration 36.0.2

I've solved it myself.

Ah what was it?

Instead of "event.cancel();", I used "event.setAmount(0.0);" so the event process of Regeneration won't be affected.

hmmmm, my bad. I didn't check it out before closing this issue
Same errors still occurred after that......

I found the real problem finally...
I'm updating player data in tick event and that's the very thing affecting the Regeneration process.
Now I move the updateData into another event and it works fine.
Sorry.