if you really want to use a self-signed certificate, generate it with:
DOMAIN="example-domain.com"
mkdir -p /etc/openpanel/caddy/ssl/custom/$DOMAIN
openssl genrsa -out /etc/openpanel/caddy/ssl/custom/$DOMAIN/$DOMAIN.key 2048
openssl req -new -x509 -key /etc/openpanel/caddy/ssl/custom/$DOMAIN/$DOMAIN.key -out /etc/openpanel/caddy/ssl/custom/$DOMAIN/$DOMAIN.crt -days 365
and later when you do want to use LE, remove self-signed files and restart panels:
rm -rf /etc/openpanel/caddy/ssl/custom/$DOMAIN/ && service admin restart && docker restart openpanel