I got your details here and thank you so much for it. I tried uploading from Chrome and firefox and didn’t have any issue at all. I think the plugin is taking js file cache of older version. Can you please clear you browser cache and try again ?
I had the same error pop up with the latest version of the plugin. Reverted back to an older version which always worked for me, and now even that one is not uploading fonts.
Please post solution here. Looks like you have enough samples to confirm the issue.