If you disable multiple uploads and cancel upload from Dashboard for only one item (press x on icon), you can't add new items.
Message is 'Cannot add new files: already uploading.'
Cancel from the status bar is working and Cancel All is working.
Can reproduce with allowMultipleUploads: false.
@goto-bus-stop, would this issue be fixed by your cancellation WIP?
No, for this I think we need to reset the allowNewUploads state value on cancel-all