I just logged in and was greeted by this:

That's not the full list, there was about 50+ toys suddenly added to my collection, according to the addon.
Obviously, I don't own these toys, even though they are now marked as collected in the addon database.
Manually refreshing the collection does not fix the problem.
My previous login on this character was just this morning, and everything was fine.

Just to add more info: the recent update did not fix the problem, unfortunately. Many of uncollected toys are still marked as collected by the addon, see the screenshot:

Is there any way to rebuild the database without deleting addon settings?

We've seen this happen to a couple users but it doesn't appear to be consistent. Toys are one of the object classes we cache in the client, so you can effectively remove just the Toy section of your saved variables file. Once you log in, your toys will be cached again and that should show them as uncollected again. Make sure to do this while you are not logged in to your account.

1. Open \World of Warcraft\_retail\WTF\Account\YOURACCOUNTNAME\SavedVariables\AllTheThings.lua in a plain-text editor like notepad or notepad++
2. Search for ["CollectedToys"]
3. Delete the entire CollectedToys section

That solved the problem, thanks.

And today it has happened again as soon as I logged in on one of the characters.

This should now be fixed with 3fbaed6. Will leave this issue open for a while longer until confirmation of resolution or issue inactivity, whichever comes first.

Do I need to delete the CollectedToys section again, or is refreshing the collection enough?

You'll need to clear your ATT cache for CollectedToys, yes.

I added the fix to my copy of ATT and cleared the cache file section. Everything is back to normal now, but let's wait for a day or two to make sure the issue doesn't resurface.

The problem seems to be fixed now, thanks!

Happened again a minute ago with ATT 1.9.9c (latest from the Twitch client).

I also realized that in the config file, the toys that have been erroneously added are marked as "true" instead of "1" (see screenshot) : for example, the Carved Ankoan Charm (id:170155) that I don't have is marked as "true", while the Verdant Throwing Sphere (id: 155184) that I have unlocked is marked as "1".

the 1.9.9c release did not include the fix for this. The next release will have this included; alternatively, you can download the latest in-development version directly from GIT and then follow the steps noted above to clear the CollectedToys array, resetting the collection state of any erroneously marked toys.