Try run these from your server:
head -c 50 /dev/urandom | base64 | curl -v -d @- -X POST -i -m 10 -H 'Expect:' https://acme-v02.api.letsencrypt.org/acme/new-acct
vs
head -c 30000 /dev/urandom | base64 | curl -v -d @- -X POST -i -m 10 -H 'Expect:' https://acme-v02.api.letsencrypt.org/acme/new-acct
Let me know if the second one times out (it shouldn’t, they should both give HTTP 415).
Previously: