Now under the help of @Chocobozzz he found the problem:
-
I modified the nginx setting of @api proxy header set from $host to $host:443 thinking it would solve the problem of getting error of « Cannot retrieve OAuth Client credentials: Getting client tokens for host peertube.slat.org is forbidden. Ensure you have correctly configured PeerTube (config/ directory), in particular the « webserver » section. ». However this change made my instance unable to get video from other instances and also unable to follow them. Removing it solves this problem.
-
For the « Cannot retrieve OAuth client » problem we found that it was caused by my changing of nginx settings to suggested one, and restarted nginx, but didn’t restart peertube service. So the host it got and it expected was inconsistent. After restarting peertube service it is solved. (Reference)
For your reference.