Really a painstaking, and RIDICULOUS error...Here are my debugging steps:
1. The error is registered at the line after \printbibliography. I initially thought it might be due to there is some hid symbol on that line. So I deleted that line, no use. Further deleted all contents below till \end{document}, same error.
2. Searching on web some ppl said such error could be due to there are symbols in the bib file which are unrecognizable to Latex. Then I searched \u233c in my bib (you have to use Extended Mode if using Notepad++) and I did found that symbol and deleted all of them. But still, such change made no difference.
3. I then tried the following steps:
a. deleting all contents in the bib file.
b. deleting all contents in the tex file between \start{document} and \printbibiography.
c. adding \DeclareUnicodeCharacter{223c}{{}}.
All to no avial! This drove me crazy now...
4. Then I realized that I once read that the derived files could make the errors persistent. So I threw all derived file (.aux, .bbl, etc) into Recycle Bin. The error was gone!!!
