Update, because I wanted to test a bit more first.
The black wire had not unseated itself, but when I removed it I saw this. A couple of the strands had fallen out/been ruined when the electrician installed it.
Anyway, I snipped off the last 1/4", reattached the wire, and was able to fill 2/3 of the battery on the Leaf yesterday at full amps with the black wire only feeling mildly warm instead of very hot (and it was very hot when I set charger down to 16A, previously). Breaker didn't throw. I will update this thread if this issue recurs but it looks resolved now. Thanks for all the posts!
How many times have you reset the tripped breaker and continued to use it? If a couple times, the breaker is likely OK to continue using, but if, say, 10 times, it should be replaced by a qualified person.
I think it threw 3 times, maybe 4. I know it's bad to have them trip too much so I didn't beat on it.