JabRef 3.6
windows 7 6.1 amd64
Java 1.8.0_66

Steps to reproduce:

1. load the pubmed/medline entry ID 27623067
2. make sure that the % signs in the abstract are escaped to %
3. the compilation with the actual biber gives a warning:
   Utils.pm:164> WARN - month field 'Sep' in entry 'PRR+16' is not an integer - this will probably not sort properly

This is due to (from texexchange):

Such month abbreviations are BibTeX macros and if you protect them in braces, they are not parsed as macros but as literal strings. You need, in your example:
MONTH = Sep
and not
MONTH = {Sep}

While all entries in Jabref are now in braces, this one should be not.

Removing the braces did the job. but how to do this in a 1800 entry bib-file