I downloaded the most recent version from the website yesterday and
installed it on an htc one mini (which had the play version installed before)
and on an xperia tablet z2 (which did not have the play version installed).
Both devices run Android 4.4 and on both devices i am unable to purchase
the PRO upgrade.

When I click on on Upgrade i geet a window saying:

Associate NZB 360 Pro with ....

and nothing further happens.
Please advise on how to procede further.