Hello I have only just found this out today because I use firefox and use the integrated spell check. However the spell checker in tinymce throws a few different errors for different people. I am still collecting data so don’t have an exhaustive supply of error messages yet. When I run it I get:
Error response: <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd”>