I had an azw3 file and the same thing happened. I used Converter365 azw3 to epub to convert azw3 file to epub. I just opened the file as epub and it worked perfectly. Maybe someone had a similar problem…
I am so happy to have found this thread.
I only wanted to add I tried a few self hosted solutions for displaying in the iframe and the very very best imho is Kavita.
how can you add url when amazon required to sign-in. Can you instruct me.