Error report 638907400
The error occurred during a sync. I haven't synchronised for a long time and have changed the local data a lot (deleted thousands of items and imported thousands of new ones).
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Upgrade Storage
Alternatively, if your local library has all your data as you want it, you might have better luck with a Restore to Zotero Server from the Sync->Reset pane of the Zotero preferences.
(Also be sure you emptied your trash locally after deleting those thousands of items.)