The error points to a file path that does not exist in Joomla 3, but was present in Joomla 2.5. For some reason or another, this path is being used by the system, rather than the correct, current path, which is causing the error in JCE.
I have renamed the folder so it cannot be used and this has fixed the issue. The path was libraries/joomla/form/field and I have renamed it to libraries/joomla/form/_field
Ryan Demmer
Lead Developer / CEO / CTO
Just because you're not paranoid doesn't mean everybody isn't out to get you.