Error Received error from Zotero server: Invalid 'itemKey' value 'collectionKey'.
Just loaded the new beta web library (looks great!) and I got:
Error
Received error from Zotero server: Invalid 'itemKey' value 'collectionKey'.
Error
Received error from Zotero server: Invalid 'itemKey' value 'collectionKey'.
From what I can tell this only happens if you change the library while the copy is taking place (there is a small popup in the bottom left corner while it happens). As a temporary workaround just wait for this to finish before changing library. We should be able to get a fix for the next release.