Unicode usernames

The answer is yes.
It seems that only non-latin usernames have problems in my case.

(This api_key was immediately revoked by me, so there is no problem with leaking it.)

I’m curious to know if this particular site is installed in a different manner, perhaps another proxy in front of it?

It is indeed behind the proxy. I’m not sure, maybe that had an impact?

I suspect the proxy may be the cause.

Can you send the request direct to the real server’s IP and see what result you get back?

That’ll let us know if it’s Cloudflare interfering.