I was having the same problem and appears to be fixed after I whitelisted make’s IP addresses for my region in my firewall.
I suspect for others experiencing this too, that there is a high chance they are getting html returned because of Cloudflare or an equivalent service running a check, which is causing the error.
You can find the IP addresses to whitelist for your region here: https://www.make.com/en/help/connections/allowing-connections-to-and-from-make-ip-addresses