Thanks for your quick answer.
I have sold my forum recently that was hosted on IPB. I was so frustrated with it and it is becoming worse with each update... Since I plan to start a new forum I am currently testing Flarum but its development is pretty slow. The only advantage over Discourse is that it can be installed on CentOS 6. If I remember correctly Discourse requirement is CentOS 7..
Today I have found out about nodeBB and those three forums "feel" very similar (Discourse, NodeBB and Flarum), although at first NodeBB seems more advanced.
I will try it for sure..