It sounds as though the database is a single file on the network, and
everyone is accessing it. You can get that message, if someone didn't close
it properly.
With no one using the mdb, go to the folder and look for a file with the
same name but a ldb extension. If it's there, delete it. Then open the mdb
and go to Tools, Database Utilities, and Compact and Repair.
You should really split the mdb for multi-users. Leave the backend (just
the tables/relationships) on the server, and give each user a copy of the
frontend (all other objects with linked tables; linked to the backend) on
their computer. You'd also leave the mdw in the same folder as the backend.
Since the database is secure, you shouldn't use the Database Splitter
wizard, as that will leave the backend unsecured. Instead, split it
manually
http://www.jmwild.com/SplitSecure.htm
Having said all that, I find it odd that 'Admin' is showing up in that
error. That is the Access username, and if the database is secured
properly, then no one should be logging in as Admin. Are you sure that it
is secured? You can test this, by ensuring you are joined to the standard
system.mdw workgroup that ships with Access (Tools, Security, Workgroup
Administrator). Then open Windows Explorer and double-click the mdb file.
If it's secured, then you shouldn't be able to even open it.