When Nginx acts as a reverse proxy, a 502 error usually indicates that there is a problem with the back-end server (such as the upstream server), not with Nginx itself. To solve this problem, you can take the following steps:

1. Check the status of the upstream server: First, make sure that the upstream server is running and can handle requests normally. You can try to access the URL of the upstream server directly to check whether it can respond normally.

2. Check the log of the upstream server: Check the log file of the upstream server to find any error messages related to the 502 error. This may involve accessing the log file or contacting the administrator of the upstream server.

3. Check the configuration of Nginx: Make sure that the configuration of Nginx is correct and the connection with the upstream server is set correctly. Pay special attention to whether the upstream server address and port number in the proxy settings are correct.

4. Check the firewall and network settings: Make sure that the firewall or other network settings will not prevent the connection between Nginx and the upstream server.

5. Restart Nginx and upstream servers: Sometimes, restarting these services can solve the problem. Be sure to back up the configuration file before restarting, check the relevant documents and know how to restart correctly.

6. Check other possible problems: such as insufficient upstream server resources, excessive requests and other problems may also lead to 502 errors. According to the specific situation, you can take corresponding measures to solve these problems.