Hi,
although the error message sounds a bit obscure, it’s indicating that you have an extra }-brace somewhere in your abstract in one entry without ab opening bracket {. This creates invalid bibtex.
I hope you saved a bit more often, so maybe it’s just the last added entry.
You could try to open the file in an advanced text editor (e.g. Notepad++ on windows) that highlights matching braces and check manually if you spot it.