I have looked into the response that my server replies with - and everything seems ok - the app was also working previously - and I have looked into similar topics here on this forum only to found that most were missing an intermediary response or some kind of other issue.
Now I'm wondering if the issue might be a timeout - as sometimes the reply takes 2 seconds to be given
I also have some doubts on the content of the reply itself - as in the docs it does not state what kind of JSON it is e.g. JSON allows unquoted numbers and booleans (and there's both a number and boolean there)
Regardless here's the initial support ticket I wrote to Shopify support - only to find out after 4/5 emails that they don't give that kind of support anymore via email.. *sigh*