Hmm, I know someone who installed gimp-cce a lot of time without problems, in /opt. And I know someone else (me) who installed gimp-cce in /usr/local without problems.
In both cases eventually "something" went wrong and the easiest way to get back to a working installation was to completely uninstall everything from /usr/local and from /opt and instead install in a prefix in the home folder.
Just to make sure, did you follow the instructions on this page regarding setting up the prefix? http://ninedegreesbelow.com/photography/build-gimp-in-prefix-for-artists.html#prefix
Specifically this line:
as per this discussion? https://mail.gnome.org/archives/gimp-developer-list/2015-February/msg00052.html
I agree it's strange and very annoying to have a setup that works just fine suddenly stop working. Perhaps there was a problem in how the last install was done, in which case completely uninstalling and deleting any remnant files from the install folder is the first step. It's a lot easier to do this when installing in a prefix in the home folder than it is when installing in /usr/local or in /opt.
Hope some of the above helps!