Thread:Extension talk:CategoryTree/Problem loading data. Please wait a moment and try again/reply

There is a bug logged directly relevant to this: T44938 Expanding widgets fails for numerical-only categories with "Problem loading data", no request sent

The problems is still present in MW 1.24.0

If you look at this bug log, a user known as Schnark has noted that there is a problem with some Javascript code that turns numeric categories into 'null' titles. This situation would tie in with your debugging backtrace message.

Of course there could be more to your problem than this.