I checked it and it shows it's because of the auth failure.
Here is the solution if someone needs:
You can add a new website with your subdomain and use reverse proxy to http://localhost:11334
Or you can use SSH tunnel to Access Rspamd Webui via http://localhost:8080:
ssh -L 8080:localhost:11334 email@example.com -N
The password for Rspamd Webui can be found on /etc/rspamd/passwd file.