Invalid iv in the message

I then went on 1password.com, and in my Guillaume vault, an item was left. That item was NOT showing in 1Password 8 for Mac.
So I moved that item to another vault (Personal), and now, I can successfully list both my Guillaume and Personal vaults!
I moved that item back into Guillaume, and voilà. I can now list all vaults using the CLI, and that item works too. I guess moving it from 1password.com fixed the problem with that item.

FYI:
That item was NOT My Item Name, it was something else.
That culprit item had:
- last modified: February 28, 2016 5:00 PM
- created: November 24, 2009 9:10 AM

So for anyone with a similar problem:
- Create a new vault
- Using a native 1Password client (Mac or Windows): nove all items from the problematic vault into that new vault
- Go on 1Password.com, and look into the problematic vault; you should see 1+ other items. Move them into the new vault from the web interface. You should now see all items, including those problematic items, in the new vault.
- Using a native client: nove all items from the new vault back into the problematic vault
- Delete the now empty new vault
The problematic vault should now be OK. Yay.