Maybe you have accented chars in labels, but the file is not being served as UTF8. That breaks the javascript application. One way to overcome this if you cannot modify mysql collation is to change in autoconfig-buttons-freepbx.sh the following:
mysql -EB -u $DBUSER -p$DBPASS -h $DBHOST $DBNAME -e
"SET NAMES utf8; SELECT device AS channel,type,if(type<>'trunk',exten,'') AS extension,\
Notice the bold text, that is what you need to add. Or you can refrain on using accented chars in labels, that might probably work funny in caller id displays.