I have the same problem on an iPhone 8 plus, which automatically upgraded iPassword to v.7 last night. The dropbox vault is an agilekeychain folder. 1Password 6 is installed on an iMac and iPad Pro.
Welcome to the forum. Odd that it took so long for you to get the v7 update. It was released November of 2017. In any event, 1Password 7 requires a newer sync format, and no longer supports Agile Keychain. You can read about how to upgrade the sync file using your Mac here:
I hope that helps. Should you have any other questions or concerns, please feel free to ask.
P.S. 1Password 6 has been retired and is no longer being developed. You can read about the upgrade options for v7 here: