Thanks for confirming Dawid.
After testing some more, I can reproduce the issue more frequently if I do the following:
# start game
# launch some activity (such as an interstitial ad)
# close that activity and return to game
# at this point, have CCLabelTTF display some text ==> here the text is just black boxes
At this point I think either the texture is being lost or it was never being created in the first place.
Anybody else come across this? Any ideas how to solve?