This gives me the same error.
Also I tried first command once more (without removing the existing certificate):
certbot certonly --standalone --preferred-challenges "http-01" --http-01-port 50080 --http-01-address 192.168.1.101 --agree-tos --register-unsafely-without-email -d "mydomainname"
Asked me what to do (to keep the existing one or to renew). I selected option number 2 and it worked fine. Just added --dry-run at the end and got the same error with port 50080.