When I try to create lessons by copy/pasting longer texts (15000 words), Lingq would create the first two lessons (2500 words each) and I can read them just fine. However, while it will show lessons 3,4 etc in my lessons feed, I cannot open those lessons. 
Error message: “There was an error opening this lesson. We have been notified and will investigate this issue. You will be redirected to the home page where you can try opening another lesson”.
I have had the same issue for weeks now while learning Japanese. One workaround is to copy paste the whole text again in the first lesson that LingQ created and open it. This generates a huge lesson with the entire text, then you can just delete the other lessons. This works for me at least.
Sorry about that, we are familiar with the issue and we are trying to figure it out and have it fixed. Thanks for your patience!
Any fix in sight? This is really annoying!
Unfortunately we can’t fix it in LingQ 4.0, but it is fixed and is working fine on LingQ 5.0. Will be fine after the launch, or if you are in Beta, you can import through Beta without issues.