Can't Update Manager Users in 1.0.6

Successfully upgraded to 1.0.6 from 1.0.3. Everything is working great EXCEPT now for some reason we can't update Manager Users. Users can be deleted but we can't add new ones or change passwords or any other details for ones already in place. Suggestions?

What happens when you try?

Check the Javascript error console, as well as the Manager's Reports -> System Events log.

Nothing shows up in the System Events log but here's what came up for the JS error console:

From FF:
Timestamp: 8/15/12 12:46:38 PM
Error: TypeError: document.userform.save is undefined
Source File: http://localhost:8888/hdfevents2012/manager/index.php?a=12&id=1
Line: 1

From Chrome:
Uncaught TypeError: Cannot call method 'click' of undefined index.php:216
onclick index.php:216


Additional context/information:
I have two local installations in place now for testing. One is the site that was upgraded from 1.0.3 to 1.0.6 and the other is a completely fresh installation of 1.0.6. The fresh installation works completely.
The upgraded version has a few issues:

• Cannot update Manager Users
• Tools > Configuration - Is missing the 'File Manager' tab
• Tools > Configuration > Interface & Features -> Missing the settings after 'Path to CSS File'

I tried reloading the manager folder and the assets folder and have found that the above issues disappear when I replace the folder 'cache'. After replacing this folder, I can update the Manager Users BUT if I update Tools > Configuration in anyway and click save then all of the above issues again appear including being unable to update the Manager Users.

I am very confused as to what could be causing this exactly.

Update:

I found that by disabling FCKEditor under Elements > Manage Elements > Plugins all the problems disappear for some reason.

Any idea as to why? It works consistently in my testing.

Sounds like Javascript conflicts. I don't think too many people use FCKEditor any more, so this may not have been noticed much before now.