It's a bug
i attach 3 patch file, download the specific version and apply it
How to apply (only for linux)
go to your moodle root folder and follow:
cd mod/simplecertificate
cp <PATCH FILE> .
patch -p1 <PATCH FILE>
rm <PATH FILE>
For example (moodle 2.7)
Donwload moodle27.pacth and save in my home folder
cd /var/www/moodle
cd mod/simplecertificate
cp ~/moodle27.pacth .
patch -p1 moodle27.pacth
rm moodle27.pacth
You can download and install the fixed version too (2014070500)