Check the workaround in
http://forums.zotero.org/discussion/20351/
"Possible workaround: The problem might just be that
there's a delay before uploaded files are available, which would be a
problem for the server verification but possibly not for a normal Zotero
file sync. You can try going to about:config in the Firefox address bar
(or clicking "Open about:config" from the Advanced pane of the
preferences in Standalone) and setting
extensions.zotero.sync.storage.verified to true, which will override the
server verification and let you try to sync. No guarantees, obviously."
The setting in Zotero is https://www.box.com/dav/zotero.
