There’s been a change at some point to the JSON format that Let’s Encrypt returns challenges in.

If you have an “old” installation of letsencrypt.sh that pre-dates v0.2.0, letsencrypt.sh is probably doing this:

$ ./letsencrypt.sh -c
# INFO: Using main config file /home/letsencrypt/letsencrypt.sh/config.sh
Processing somedomain.netcalibre.net
 + Checking domain name(s) of existing cert... unchanged.
 + Checking expire date of existing cert...
 + Valid till Jun 22 16:24:00 2016 GMT (Less than 30 days). Renewing!
 + Signing domains...
 + Generating signing request...
 + Requesting challenge for somedomain.netcalibre.net...
$

and then it silently exits.

Update it from git, move your config file to the new location:

git pull
mv config.sh config
./letsencrypt.sh -c