Generally, when one sets up security, a new mdw file is created. Then a shortcut needs to be created to launch the Access database. The properties for the shortcut would look something like this "\\path\msaccess.exe" "\\path\YourDb.mdb" /wrkgrp "\\path\YourWorkgroup.mdw".
However, if that is not how you launch it, then Access uses the System.mdw file by default. I suspect when security was setup, whoever set it up, didn't create a new workgroup. To get around the problem, without fixing it, is to create a shortcut and set its properties like this: "\\path\msaccess.exe" "\\path\YourDb.mdb" /wrkgrp "\\path\System.mdw". Then launch the shortcut. If it works, then that tells me that Access overwrote the System registry so that it points to the new mdw file you created with the wizard rather than pointing to system.mdw.
If the above works and you want to fix the problem so it works like it did before, you need to mess with the System Registry. The system registry should now be pointing to the new workgroup you created. So search the System registry for the name of the new workgroup and change it to system.mdw (or whatever workgroup you were using). To search the system registry, type regedit via the Run command. But be very careful when messing with the registry, you could cause some major problems. I suggest backing up the registry prior to modifying it.