could not connect to fulfillment server mac

I've already had an account with Adobe and recently downloaded ADE 3.0 for Mac (iOS 10.10.3), but when I'm now trying to download an ebook I'm getting an error that ADE could not connect to the fulfillment server.

I already had a look at the firewall and enabled ADE also for inbound connections, made sure that I'm logged in as admin and also tried the following link as suggested in other threads: http://adeactivate.adobe.com/. When I open it, I'm getting "Welcome to Digital Editions".

What else could be the reason I'm still getting the error?