Big bug/problem - no skill works!
I just tried strengthening the only of my learned skills that wasn't maxed out, first on its own, then through the "Strengthen skills" button - but neither works. The owl shows up to tell me "Lesson Practice: Complete a lesson that includes words you've learned", but after I click "Practice", the owl is gone - but no exercise shows up in its stead. The middle of the screen is simply empty, and the "skip" and "check" buttons are greyed out.
I tried "strengthening" the skills that were maxed out already, with the same results.
I've tried starting one of the new skills I have available - but that didn't work either, although in a different way. Simply, clicking "Begin" does nothing. The loading sign in the O of "duolingo" spins for a while, but then it stops, and nothing happens.
Do you have any idea what's wrong and how to fix it?
Update: the bug seems to have been fixed after I've purchased and started the "Christmas" bonus skill. It worked normally, and after I've completed the first level of Christmas, I was able to strengthen my old skills and to start other new ones.
Weird, but at least it's fixed. :)