Anonymous downloads not working with Tribler 6.3.4 - why?

Hello.

First of all, a great thanks to the developers for bringing this piece of software ! Highly appreciated.

Now the problem :
I installed Tribler 6.3.4 on a Linux pc last week. I have started eight different downloads, setting various anonymity levels.
All non-anonymous torrents (4 of them) have downloaded successfully. But all anonymous downloads have still at 0%, showing ‘building circuits’, including the “Automatic test of Tribler Anonymous downloading - I will disappear when done”.

What may cause this problem and what can I do so help solving it ?

Correct only from later versions for sure Tribler 6.5.?