This error occurs if the GN4 database is badly installed, or so badly corrupted that the system cannot find the default schema.

To correct this error

1.Make sure you're not connecting to a wrong database. Check the system settings as explained in Configure database connection and verify that the database is the expected one. If the database is not the right one, change the configuration data as explained in Configure database connection, reset IIS and then retry to connect.

2.If the database is the right one, you may want to start to search for the last valid backup of the GN4 schemas. Load schemas in database as explained in Importing configuration files in database (quick chart) - look at the "Import schema files" paragraph.

3.If the step 2 does not resolve the problem, you may want to start to search for the last valid backup. If you find it, restore it - you will loose all data stored in the database after the last backup, but if the backup is a valid one, you have a chance to restore the system to its full functionality.

4.If the step 3 does not resolve the problem, you may want to start the reinstalling of the system from scratch.