That's just a file name. Double slashes are ignored but even if not would not interfere with curl comms. Something else odd is happening.
Does the problem repeat at the same spot each time? If so, I'd guess pfsense interference first but only because I have no better idea 
Can you just do curl's to that. These will give http 405 but curl itself should not fail:
curl -I https://acme-staging-v02.api.letsencrypt.org/acme/new-acct