While "strengthening skills", at the end of 19 exercises, I clicked on "check" (or maybe "continue"), and the "o" in duolingo rotated and and rotated and rotated without giving me an answer. After a couple of hours wait, I clicked "Quit", and was asked if I really wanted to, that all progress would be lost. So I cancelled and the "o" in duolingo rotated and and rotated and rotated without giving me an answer. I could still connect to duolingo using another browser window, but my 19 exercises were lost.