Blank screens

I have been using this app (paid) on an iPhone 4 (iOS 7) without problems for about 6 months, now. About 5 days ago, I started experiencing problems in the form of various blank screens when tapping buttons (e.g. About screen), and it fails to log into the account I have. All tracked items appear like question marks.

Any ideas on what could be the cause?
Thanks,
Evelyne