I bought an app called Topo Maps in 2010 for $7.99 for my iPhone using the only Apple ID I have ever had. When I use "Get Info" for the app in iTunes, the app is said to have been digitally signed by that Apple ID. There is an available update for this app. However when I try to update it, I get the same message as the original poster of this thread. I have tried deleting the app from both the iPhone and iTunes and reinstalling (using the local file retrieved from the Trash since I cannot download the app at this pointc), clearing out all the relevant caches, signing in and out of the store, etc. with no success.
Most significantly, there is no way to contact any Apple support resource to solve this problem. All links lead to a support page related to eligable hardware. Since the app was originally bought in 2010, the hardware is well out of warranty. I have the transaction number, date I bought the app, and proof that I bought it using my Apple ID but I cannot find a way to contact a person or even an automated resource to solve this problem--which is a problem with their database, not with my system or account. Very frustrating, as this should be a simple problem to solve.
Notably, there are close to 1500 views of this discussion, so I know that many people are having problems like this. I have not seen any clear solutions for it and would welcome any ideas.