@antonpa, I think the same thing happened to me. The app was asking for a monthly payment even though I bought it with a one-time payment years ago.

To activate it, you have to select the "I'm already subscribed" option, and then "forgot your password" with the email you used to buy the app. Then you log in with your email and password, and that's it .
I'm glad the developer is taking care of early adopters who paid for the app. Other apps like Paste have forced one-time payment users to switch to a subscription, which I doubt is legal.