Testing payments: after purchase the Pro access is still not unlocked

Hi there, it's been about 3 hours of experimenting and I still cannot resolve the problem.

I'm testing the payment with PayPal
sandbox. I want to enable blogging on my multisite only to those sites/users who pay, but when I purchase, say, one year membership, I still cannot access dashboard and the following strange text is written along with the output:

Last Payment Date: None yet with this subscription (only initial separate single payment has been made, or you've recently modified your subscription)

Expected behaviour would be allowing users who pay to access the dashboard.