Most likely causes the fck path setting sin web.config may not be correct. Was this an upgrade? Did you try to keeop your old web.config file? If so get the new one and transfer any custom settings to it.

Only other common cause is if not all the files got uploaded. I would upload the /ClientScript folder again watching carefully in your ftp client for transfer errors and retry until all the files make it, then clear your browser cache.

Hope it helps,

Joe