Exploring acme.sh log file reveals the following error:
Using config home:/shared/letsencrypt
Installing from online archive.
Downloading https://github.com/Neilpang/acme.sh/archive/master.tar.gz
GET
url='https://github.com/Neilpang/acme.sh/archive/master.tar.gz'
timeout
_CURL='curl -L --silent --dump-header /shared/letsencrypt/http.header '
ret='0'
Extracting master.tar.gz
Extraction error.
Upgrade failed!