@antonpa, I think the same thing has happened to me as you. The app asked me for a monthly payment despite having bought it with a one-time payment years ago.
To activate it you must select the option "I am already subscribed", and then "I forgot my password" with the email we used to purchase the app. Then you log in with an email password and that's it .
I'm glad that the developer takes care of the early adopters who pay for the app. Other apps like Paste have forced one-time payment users to move to subscription, which I doubt is legal.