I had this problem too, and it wasn't anywhere near the line number given in the logs.
Building on Leah's answer, if you press Q, then look in the logs, you'll see the message:
I've inserted something that you may have forgotten.
(See the <inserted text> above.)
With luck, this will get me unwedged. But if you
really didn't forget anything, try typing `2' now; then
my insertion and my current dilemma will both disappear.
So on the next run, try pressing 2 at the error prompt, which tells latex to ignore the problem, and continue, instead of trying to fix it. In doing so, it'll print out the last bit of text it read (near the problem), and that should give you a clue where the problem is (you might need to do this a couple of times).
Turns out my problem was on line 366 of my
.bib file, and no-where near line 20 as reported. It was caused by a
~ in a url that hadn't been converted by the zotero exporter. Deleting that line removed the error.