https://trades.roblox.com/v1/users/10307570/can-trade-with
I just tested out this endpoint, and it works perfectly fine. I am pretty sure it’s a problem with that proxy.
I would recommend using Google Apps Script as a proxy, as demonstrated by @TheNexusAvenger in this tutorial. It is a completely free option, and is extremely simple to use and set up. I have personally used it quite a bit, and it works like a charm, besides that fact that is has some request limits.
Hope you find a solution that works for you!