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I got a solution :) I use Safari for Steemit on the Macbook. But Dtube does not function with Safari. So I downloaded Firefox and woahhh, the upload works ;)

thank you i test it your method with chrome

not working with chrome nor mozilla... very encouraging to start using another platform expect for youtube...

Thank you!!! I've been having this issue with Chrome on Windows 10 and after I read your post I immediately downloaded Firefox, and the upload works!!! No error when uploading the video OR thumbnail!!!

my old macbook would not work with firefox at all.