Hmm… Systems using openssl 1.0.2 won't work with the default chain, which (last I checked) the Let's Encrypt API endpoint is still using. At some point they're said the plan was to move to the ISRG Root X1 rooted chain, but they haven't said yet when that will be (though we have a thread or two asking about it).
You might need to use one of the Openssl 1.0.2 workarounds to let your system connect to the Let's Encrypt API in the meantime.