"This skill is out of XP for today!"
So I'm on my Korean tree (my least gud language) and this appears:
I'd done the Practice for this particular skill about 2/3 times and only got 2 - 6 xp each time before timing out.
I haven't noticed this on my German/Spanish trees because I tend to use the general "Practice Mode" and am still going through the 'new' Spanish tree.
Did I miss an announcement? Is Duolingo further trying to control how we choose to learn? I know how well I know a skill (or don't in this case). I know what I need to practise and focus on. Why take that away from me?