All right, I actually found what is the problem. It's related to the type of user. Initially the user was part of Registered users group, then I moved it to Author and now I get the page displayed properly!

Hmmm...

I found it's permission related after reading this on Internet

==============

Fatal error: Call to a member function get() on a non-object in
\plugins\editors\tinymce.php on line 433

error because the object that the statement try to call is not an object,
on line 422 of this file call to the JEditor::getButtons()

on line 261 of the editor.php the code is not right

$result[] = $plugin->onDisplay($editor);

because the register doesn't have the right to have the image button, so the
function $plugin->onDispaly($editor) when the $plugin is 'plgButtonImage'
will return the null this will lead to the error above.

solution is replace the line

$result[] = $plugin->onDisplay($editor);

with

$temp = $plugin->onDisplay($editor);
if ($temp) {
   $result[] = $temp;
}

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I didn't implement the code change I just changed the role of that user, but still I don't want all my registered users to be Authors, what should I do? Any ideas...