I have seen this error a LOT since upgrading to 8.5.0. It seems to be triggered more by the Notes 8.5.0 client than the server. For some reason the Notes client does not release a lock on the database after it is closed.
e.g. I can create a database, open it, close it, and then try to delete it and I am quite likely to get the message. The Admin console will confirm that nobody has the database open.
My suggestion would be to upgrade to Notes 8.5.1. I'm not sure a server upgrade is needed based upon my suspicions it is a Notes 8.5.0 bug.