Hello, some people have reported this before but I am still unable to determine that TinyMCE editor is being loaded. As you already guessed the culprit seems to be one of the hardening options, specifically the one that denies public access to PHP files inside the includes directory.
I implemented a tool in the hardening page that you can use to whitelist specific file names, so if you have applied the blacklisting you can use this tool to ignore the block over the file(s) used to load the TinyMCE editor. This tool was implemented with commit 1274945 [1] which will be released with the next version of the plugin, or you can download the development version from here [2] if you want to use it from now.
[1] https://plugins.trac.www.ads-software.com/changeset/1274945
[2] https://downloads.www.ads-software.com/plugin/sucuri-scanner.zip