update—
I followed the steps to reinstall the plugin:
Uninstall WP Mail SMTP option in WP Mail SMTP > Settings > Misc and then deleting the plugin.
I finally could access the settings page.
However, I encountered the problem again. I save settings with client ID and client secret, and then I got the same issue with a 502 error. In this time, I cannot see any error logs on “Debug Events”.
