Error 502

Hi, I'm having a problem with Cloudfare. Let me explain: I have my domain on Cloudfare configured with a subdomain that points to a DDN on my MikroTik. I have a reverse proxy with Nginx on the origin server, and from there to my backend. It turns out that since Cloudfare maintenance, some clients work perfectly, while others are returning a 502 error, which doesn't make much sense to me. I've already cleared the cache on both Cloudfare and the machine. The SSL certificates are from Cloudfare; I don't know where else to look.