what is the problem in the song of roland?

Ganelon is enraged, thinking that Roland has nominated him for this dangerous mission in an attempt to be rid of him for good. Ganelon has long been jealous of Roland, and on his diplomatic mission he plots with the pagans, telling them that they could ambush Charlemagne's rearguard as Charlemagne leaves Spain. Roland will undoubtedly lead the rearguard, and Ganelon promises that with Roland dead Charlemagne will lose the will to fight.

I'm not sure what you mean by problem but that sets up the conflict. Please take a look at the GradeSaver summary, it should help.