Most likely when you upgraded you tried to to keep your previous Web.config and therefore are missing some important new settings related to FCKeditor. In this case I recommend get the new Web.config to make sure you aren't misisng anything and restore customizations from your old one.
If its a clean install then most likely you had ftp errors and not all the files got uploaded. Often I find it works better to ftp the Data, ClientScript, and Setup folders separate from the rest instead of trying to upload all at once.
After resolving the above it is also advisable to clear temp files from your browser.
Hope it helps,