I was importing my unmanaged solution then the error has occurred.
I was using a new UI. After spending some unpleasant time researching and trying different things I came across this post in the Hosk’s blog:
Thank you very much, Hosk, you are truly marvelous!
I realized that I have to switch to the Classic interface to get a meaningful error.
I did. I tried to import the solution using the Classic interface. I was missing one view in my solution. It took me 5 minutes to fix them re-import.