Upgraded to Zotero 7.0.15 and now have items missing from my library... papers I've had for years have just disappeared (online and in the app) - is this related or is it another mystery?
An app update would never cause you to lose items, no.
A handful of items were deleted in your library yesterday, and hundreds more were deleted on February 21. If you didn't do this on purpose, you can check your security settings to see if you maybe left yourself logged in on some shared computer.
If you had the items on your current computer within the last couple days and just synced these deletions (made elsewhere) down, you may be able to restore from one of the automatic backups.
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A handful of items were deleted in your library yesterday, and hundreds more were deleted on February 21. If you didn't do this on purpose, you can check your security settings to see if you maybe left yourself logged in on some shared computer.
If you had the items on your current computer within the last couple days and just synced these deletions (made elsewhere) down, you may be able to restore from one of the automatic backups.