Note the latest version of MODx' TinyMCE is outdated. See its changelog: Update TinyMCE to 3.5.4.1
Whereas the latest official TinyMCE is 4.0.22 or so.
Nonetheless, I think this instance of code is still valid in the MODx version.
valid_elements: "em/i";
// slash instead of comma
Which version are you using?
Your problem is probably, duplication of code somewhere.
I just checked this xconfig file in question, and I saw under
valid_elements
+'em/i[class|dir<ltr?rtl|id|lang|onclick|ondblclick|onkeydown|onkeypress'
Once again, what version are you using?