I tried to upvote this question on Meta Stack Overflow using the Stack Exchange iOS App Version 18.104.22.168 (current version right now). I have a 4th generation iPad with iOS 8.0.
Got an error message stating:
"Could not perform this request with your account. Please try logging back in. If this persists, let us know on meta."
Dutifully, I then tried logging off and in again, with the same results. I was able to upvote the question by using the mobile app on Safari, no errors at all.
I also can upvote other questions and answers in SO using the iOS app, so this might be related to the specific question/site.
Thanks for all the hard work! The app is already great on iPad!