Had this extremely annoying problem, since of course it is nigh impossible to find unless you know what it is about. I ended up having to remove all citations and adding them one by one to find the problem. Searching for unicode characters in either the LaTeX or .bib file won’t help!

This problem is related to this: https://tex.stackexchange.com/questions/83440/inputenc-error-unicode-char-u8-not-set-up-for-use-with-latex Stack Exchange question.

The problem was this:

Llu{\’\i}s

construction in the .bib file.