This fix applies to all Ultimaker 2 series machines, and Ultimaker 3 series machines. You will need your 2mm hex head screwdriver or 2mm allen key to apply this fix.
You may notice a little slackness in one of the 4 long perimeter belts on your printer. We often receive the question: Is my belt stretched out? Does it need to be replaced?
The good news is, your belt has not been stretched out and doesn't need to be replaced.
What's happened is that something caught on your belt (your hand, the printhead shaft while doing maintenance, etc.) causing it to shift it's position on the pulley teeth. This makes half of the belt slightly too tight, and half slightly too loose. The tight half won't be very visibly different, but the loose will be.
Use your 2mm screwdriver or 2mm allen key to loosen the set screw on the pulley of the loose belt. The belt will snap itself back into the correct position, and you can tighten the set screw again.
Watch the whole belt tightening process star to finish, right here.