I upgraded from WordPress 2.5 to 2.51 and am getting a Error: TinyMCE is Undefined error when I try to create or edit a post or page.
I installed WP 2.51 fresh to a different domain (but same host) and am getting the same error.
The error does not pop up right away, though. The editor is stuck in html mode, with "visual" highlighted and when I click on "html" the error pops up.
I tried the wplang fix, the compression fixes, and just about every other fix and check I saw mentioned on the forums and the bug tracking page.
If it makes a difference, when the error comes up, it sometimes follows an error reporting that a ';' is expected in line 3.
Also, my blog is hosted via a shared server running hpanel, and I configured wordpress such that all supporting files are in a subdirectory.
The same error was reproduced via firefox, IE7, and three different computers.
For the blog that went from version 2.5 to 2.51, there was a similar problem after installing 2.5 fresh. However, I was able to circumvent it using (forgotten) advice found in the forum. I retraced my steps and read most of the same threads, and as mentioned those methods failed to work this time.