My reference list include letters from different languages, such as
á, ö, 第.
! Package inputenc Error: Keyboard character used is undefined
[Copied from deleted answer by cfr as the OP cannot currently access the account.]
Augustin's suggestion solved part of the problem. It still do not want to work with
第 (and the likes).
! Package inputenc Error: Unicode char \u8:第 not set up for use with LaTeX.
I did try to, as per one of the answers, create a new document and try again. It did not work.