Our current understanding of the issue is that it appears that the tool provided by Apple to create the apps (Xcode) stopped supporting IOS 5.1.1. Unfortunately this information wasn't clear in Apple's documentation nor is the tool enforcing this limit. As a consequence the latest update of DragonBox Algebra 5+ was not restricted to not target users of IOS 5.1.1.
We are very sorry for the inconvenience and we are looking at ways to solve this issue. In the mean time we recommend affected users to try to downgrade the app version on their device or if they are able to use the app on a device supporting iOS 6.0 or later. In doubt, do not hesitate to contact us.