It’s worth noting the error message comes from the CURL library, not nginx. Here’s a thread from the last time this came up, but the resolution was that the OP was using a malformed URL: Http Socket Crashes WebServer
The line you’ve found in the nginx logs is quite common. I’ve never found the cause of why the Xojo Web app refuses the connection, but I’ve also never seen it have an effect on a running app. I don’t honestly think it’s related to your CURL / URLConnection issue.