Upload operation when performing a space/site import is stuck preventing the import

Summary

When using the new import feature for space and site, the upload operation never completes and the Import button is not clickable.

Steps to Reproduce

1. Start a space/site import by uploading the file.

Expected Results

The file is uploaded successfully and the import can be launched.

Actual Results

The upload operation never completes leaving the Import button disabled.
The following error messages is thrown in the browser console:

Workaround

No workaround yet.