Just to confirm, have you tried uploading the custom thumbnail from another device/browser? Keep us posted.
Thanks for confirming. Which browser are you using? Could you try clearing the browser's cache & cookies to see if that fixes the issue? Here's how: https://goo.gl/1xMyUq . Let us know if this helps.
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Hmm, our Creator Support team can dive into this further with you, contact them here: https://goo.gl/BxLi8H
End of conversation
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