There is a current issue with the SSL let's encrypt that I just submitted to them to resolve (they are using an outdated version with Acme 1 instead of the newer Acme 2):
FYI - we've resolved the issue, and I can only assume anyone else that has installed Postcards should have the same issue.
The problem is indeed an issue with Let's Encrypt, and specifically:
To fix this issue, please update the docker-compose.override.yml file during builds.
Row 35 should read: image: jrcs/letsencrypt-nginx-proxy-companion:latest
This will insure that the current version for the let's encrypt proxy is called and eliminate the issue.