It turned out that my old syntax highlighter “Crayon Syntax Highlighter” does not work with new PhP version (7.4+) and the plugin is no longer maintained officially. Luckily, someone updates it and provide a zip file: https://www.axew3.com/w3/forum/?coding=dmlld3RvcGljLnBocD9mPTEwJnQ9MTU1NQ==. I also back up the updated plugin on my dropbox: https://www.dropbox.com/s/e0u8jb2oqfagv9c/crayon-syntax-highlighter.zip?dl=0
BTW, I also back up the whole website using “All-in-One WP Migration” plugin: https://www.dropbox.com/s/unpeslbs404ujof/czxttkl.com-20201207-055211-640.wpress?dl=0