It could be a few things but some possible culprits:
1) Deflector blade is dinged, bent or slightly out of adjustment, or perhaps it's not seated correctly. Deflectors are the most likely culprit when bent corners start to appear.
2) Fold rollers may be worn or could have an ink/coating/varnish/dirt buildup. A good cleaning with Baum's roller wash can't hurt.
3) It could be time for a rebuild. There are bearings, bushings and spacers which wear over time. Baum has an inexpensive 714 rebuild kit and you can probably do the work yourself if you have reasonable mechanical aptitude.
Hope that helps!