What troubleshooting steps should I take if I'm encountering SQL Server error 15404 when opening an existing database diagram?

First, verify that your login is correctly associated with the database. Then, check your permissions on the `sysdiagrams` table (often in `msdb`). If you're not the owner, try having an administrator grant you `SELECT` permission on `sysdiagrams`. If you need to modify it, `INSERT`, `UPDATE`, and `DELETE` permissions are also required.