Feature request/bug report from the Forum:

1. Copy the following in the source tab of an entry
2. Try to open the url
3. Explorer is opened (on windows)

Solution would be to trim the url field automatically

URL = {
https://doi.org/10.1175/1520-0469(2003)060 1<1655:TOSOCC>2.0.CO;2
},

http://discourse.jabref.org/t/jabref-cannot-identify-the-url-if-there-is-space-between-the-url-and-opening-and-closing-braces/1314