@avidseeker
When you use a OSS service and your server can’t access it, for example, you are in China and your server’s connections to Cloudflare R2 are blocked by the local ISP. When clients try to acquire custom avatar resources, your server has to acquire it from the OSS, but failing, then returning 500 to the clients.
In my case, setting two env vars: HTTP_PROXY and HTTPS_PROXY to a proxy server which is able to access your OSS service. If you installed Discourse with standard installation, in your app.xml it should have a field called env, add those two variables and then you can run. I used unsupported installation and use Systemd to manage Discourse, so I added two Environment parameters in the .service file.
I don’t know whether your country has network censor system. If so, I can assume you already know what to do; If not, check out your OSS service’s online status and your settings about S3.