I’ve had a similar problem and have possibly found one solution. I’d love to hear if anyone else can reproduce this…
Check the wp-config.php file and see if there are any blank lines at the end of the file. If there are, delete everything after the last character of code, clear your browser cache and try again.
It doesn’t sound like the sort of thing that would cause problems but it’s fixed 4 of my sites so far with various problems disappearing after this fix. The common symptoms I had with all sites were that a) TinyMCE Advanced was present and enabled but not displaying in the edit page and b) The default visual editor only shows html code