col.lockDateModified
My Zotero is no longer able to synchronize.
It indicates "Last error: col.lockDateModified is not a function"
It indicates "Last error: col.lockDateModified is not a function"
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Clicking on the gear button in the toolbar, then selecting "Clear Server Data" solved it for me. Make sure, before you do that, that you have a full copy of your library on one machine. Sync that machine again, then sync the other.
That worked for me, anyway. I'm not sure exactly what led to your circumstance. More information is always helpful.