It is 6.1.
I figured out which slide the problem was on, and then deleted the objects on the slide one by one till I found the offending object. It didn't have a system name or special characters, so I'm not sure why it was corrupting the slide. But I deleted it and recreated the whole slide and everything is fine now. I just don't really have an answer as to why it was messed up in the first place.
Thanks for your help!