The problem is quite common when you copy text from other sources like PDF documents into your source document. What happens is that there is some UTF-8 invisible / whitespace that you cannot see and that LaTeX is unable to interpret.
You can fix your document using sed command line program to replace all those invisible characters with nothing:
sed -i 's/\xe2\x80\x8b//g' [your_document].tex
Or in case of Nested files:
sed -i 's/\xe2\x80\x8b//g' [your_document].t2t
Fix for Nested will be published with Nested 2.0 so documents with whitespace UTF8 characters get automaticly converted.