Just recieved this when trying to open a lesson. I waited a while then opened it again and got the same error code thought I should report it.
It happened to me yesterday and today as well. The fix seemed to be pressing the OPEN button on the drop-down of the lesson rather than just clicking on the banner before the drop-down. Hope this helps 
If you notice the same issue again, can you please send lesson URL to support? Thanks!
Yeah definitely! My temporary fix hasn’t worked on all of the lessons I want to do 
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I have two lessons where this error occurs. It is the same text because I thought If I simply import it again it will work. But it didn’t.
www_lingq.com/de/learn/uk/web/reader/22101286?utm_medium=lib_feed&utm_source=my_lessons
www_lingq.com/de/learn/uk/web/reader/21258687?utm_medium=lib_feed&utm_source=my_lessons
you need to replace the _ with a dot to make it www.ling