After spending lots of time for this tinyMCE bug and trying many solutions i found out there, i finally got it working. It seems to be a problem with gzipping tinyMCE code and some servers hosting wordpress.
I solved my problem on this server by editing file : wp-includes\js\tinymce\tiny_mce_config.php , line 208 in wordpress 2.5.1
Just change:
'compress' => true
to
'compress' => false
on other blog this solution don’t work.
I have disabled all plug in and after removed cache browser tinyMCE work. Than i have abilited one plug-in time and revoce cache and i have discovered that th problem on my blog i causated with :
Post Thumb Revisited plug in
Thanks for posting this. Have spent hrs trying to track down a solution for this random bug.