After forcing the use of HTTPS for the blog backend, I encountered an error "Download failed. Problem with the SSL CA cert (path? access rights?)" when upgrading WordPress plugins. Later, I found out that it was caused by the web server itself.

To solve this issue, I installed the AMChroot module on the VPS, and set the corresponding website to compatibility mode. After that, I was able to successfully upgrade the plugins or themes.

Finally, the problem was resolved. In addition, for security reasons, I recommend setting it back to secure mode after successfully upgrading the WordPress plugins or themes to avoid cross-site attacks.