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).
Looks like you're running out of memory. If you have other programs open, restart your computer and try again with just Zotero.
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.)
Thanks, doing Restore to Zotero Server worked nicely. But I wonder if the computer really hadn't enough memory available before, I guess some 2GB of free RAM should be quite enough for this operation...
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.)