by Mateusz Szuter.
I tried to do the upgrade, but my theme isn't compatible yet.
I've found what is causing problem - when I change the default "topic 1" or the section above news forum (fill it with some description) this errors throws.
So until I change that - I can import course. When I change them, I've got error mentioned above. What's strange that when I remove description and change the topic name back to it's default, the error still happens