Do you have the option to create a brand new MODx installation on the same environment? It may help to figure out whether there is a general problem with TinyMCE RTE (on this environment) or whether there is just something “broken” in your current installation.
If there is a general problem, then please communicate the specifics of the environment (like PHP version etc.)
Do you still get these error messages: “Could not load class: TinyMCERTE from tinymcerte.”?
Make sure that the file /core/components/tinymcerte/model/tinymcerte/tinymcerte.class.php has the correct file permissions (0644) and the correct content.
Somehow the loading of this class fails. There is an off-chance that there is a problem with the autoloader (but this topic is above my head).
There is usually a PHP error log file (independent from MODx) somewhere on the server. (The exact location depends on the server setup). Maybe you can check this error log (if you find it) and see if something useful is in there.
In general, I’m running out of ideas and I’m afraid I can’t help you.
You could try that and maybe someone else (who is put off by this rather long thread) has a better idea.