Trouble with Unrealbook app have to log into App store

Sometimes, when starting up UnRealbook app on my iPad, I get a notice that I have to log into the APP Store, or "Since this is the first time you are using Unrealbook, you must log in..." I have been using it for a year now, and this has just recently started happening. I emailed the developer of the app, and while he is always very helpful, he couldn't help with this one as he said he had never heard of it happening, and suggested that I need to check with Apple. So this is my first step...Any advice or input greatly appreciated, as always. Thanks