UPDATE May 2, 2:41 PM GMT:
This issue is fixed now.

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We have found a technical issue with SFTP uploads which froze the upload queue. At the moment the software fails to handle uploaded files with non-alphanumeric symbols in their names, like “image(2).psd” (brackets) or “My image.JPG” (white space)

We are working on the fix and will update you when it is available (hopefully very soon).
For the meantime please keep all the files names alphanumeric.