First question - How much free space is on the Cache folder drive? Second, have you tried restarting holding down the Shift + Ctrl/Cmnd + Alt/Option key and deleting your preferences.
Without details it's pretty hard to suggest a solution.
The warning itself is probably no big deal. Either your cache is full or (if you use a drive other than your C:Drive for caching) it's defaulted back to your C drive as you cache, or what Rick said.
As for not being able to select anything I have the same issue until I actually click on (wake up) the application window. You know the top bar windows have that you would click on to drag the window itself to another screen or whatever? Once I click that it wakes up and i can proceed. I just assumed this had something to do with how I have AE configured personally to open on a secondary screen. Basically when I start after effects I get my splash screen on one monitor but the app itself opens on a secondary monitor & then the welcome dialogue appears back on the primary. When using AE 2014 I don't have to wake the application window after startup but I just figured annoying little problems are sort of "the norm" with AE these days. But maybe this is a bug.
Anyway if you have tried it yet...try giving AE a minute to get over it's awesome self and then try waking the app window by clicking the top bar.
Thanks for all the replies. Waiting a few minutes did work, Gutter-Fish. Afterwards, I did realize that my scratch disk was indeed below 10%. So I cleared out some space and all is well. Sorry for the negativity. But as we all know, being an Adobe user can be a very frustrating experience at times.