@David,

I tried both possibilities to mark lessons as not studied again with the same result I have already mentioned. I have the feeling that when I click on the link on the specific lesson's page that the lesson is instantly marked as studied again. I tried this a few times and after clicking on "mark as unstudied" sometimes the little window that asks you to review vocabulary when you click on a lesson appears - and the lesson is still in my studied lessons list. When I click the link to remove lessons in that list it first disappears like it should but I come back to the studied lessons page it is there again.

My OS is Windows 7 Professional and my browser is Firefox 13.0. Thank you for having a look at this issue.

Ani