Zoho Projects Webhook fails with HTTP Error 0

Zoho Projects Webhook fails with HTTP Error 0

Hello Zoho Community,
I am pulling my hair out over this one. I have setup a very basic http(s) server that always responds "ok" and code 200 to incoming GET requests. It will accept any parameters, and any path. Really, all it does is say "ok," and log the request to the server.

This service works great from web browsers, from CURL, and from postman. I've tested it in a lot of places with a lot of GET parameters.

However, when I plug this URL as a Zoho Projects Webhook, Zoho fails. Zoho complains for HTTP Error 0.
Error 0 usually means 
 - client (zoho) error 
 - client (zoho) firewall blocking or filtering request
 - request was cancelled by client code
 - other client error

Has anyone run into this error before?

Is there a blacklist at Zoho I might be on? Or do I need to be explicitly whitelisted?

I have tried Zoho with both http and https, neither succeeds.

Any input is greatly appreciated.