My iPhone keeps asking for my password, transaction failed

hello everybody,

please help me before I go crazy..

I have the "simple transfer" app, and something like a year ago,I purchased the upgraded version of it (paid like 3 $ or something).

a couple of months ago I changed my iPhone, downloaded this app again and tried to activate my upgraded version and got an error message.

since then my Apple ID and my App store keeps asking for my password and sometimes refuses to download new apps or update existing apps, I can't view my apple ID (in the Settings), and can't upgrade to the upgraded version of that app, neither free nor paying again!!!

what can I do ????

I've tried deleting the app, restoring my iPhone, change my payment method and nothing helped!!