Same from this iPhone. Tried another forum at random - Fragrant Harbour - and the same redirect to the App Store.
I have the same issue and wonder if it isn't an IOS problem rather than a Prune problem? I have been on various websites (non of them dodgy) on my iPhone and iPad and been redirected to the App Store.
The fact that it happened on 2 separate platforms and on different websites to my mind suggests it might be more to do with Apple and how cookies / tracking interacts with the App Store.
Interestingly, I can no longer seem to get PPRuNe in the App store to load it onto my iPad - does the PRuNe app still exist?