Sometimes Zotero doesn’t find the (open access) PDF for a journal article, so I just export it separately from the browser share extension. Then I drag and drop the PDF in Zotero to the right item. In the latest beta I consistently get a ‘Could not move file’ error message even though the move itself happens as expected.

The other one is likely related. Following this file move, when trying to edit the item in question there’s an error message stating that the item was modified remotely, so it would now reload. Then nothing visible happens and everything seems normal.