First, do you have any emails that work? E.g. have you confirmed your smtp setup works? What does your SMTP device say in its logs? It's likely the problem is there and not on the Splunk side of things, so checking it may tell you why this is happening.
Now on another note, I had an issue in 6.1.x or 6.2.x where I had one or two reports stop sending BOTH csv and pdf. I could pick either and it was fine, but if I picked both, it would give that error. I never resolved this because we migrated to a newer server and when I moved that report to the newer server this problem stopped.