Your error is common when a project's DB is not available (security, offline, upgraded to a newer QC version, etc.).
Can you confirm that the project's schema for ''Default_myproject_db'' (I am assuming this is the name) is actually on ORACLE server DEN-ZZORA2 ?
You may need to do a ''walk'' down your Site Admin project list to confirm which projects are expected to be where (i.e. different ORA servers).
In Site Admin, on the Site Projects tab, click on each project one-by-one, and look at the Project Details tab - here you will see the :
Database Name (default_myproject_db)
Database Server (DEN-ZZORA2)
Sometimes while migrating projects to a new server, people accidentally do some bad things:
1) move project Schema/database to new server and de-activate or drop from old DB server
2) Accidentally leave OLD project active in OLD QC server and at the same time make it active in the NEW QC server (2 QC servers pointing to the same Project DB/Schema). If this happens and the project is Upgraded in the NEW QC server, the OLD server will definitely have issues using the project DB since it is now upgraded to a different version.
You could also possibly have some of your QC projects' databases/schemas living on DIFFERENT Oracle servers. If one ORA server is down or not available, that would explain why some projects work fine and others not. (It is best practice to only use ONE Oracle server for ALL QC Project schmas AND for qcsiteadmin_db schema -- ALSO it is best practice to use the SAME schema-user password for every QC-related schema).
So, if you make a list carefully of all of the projects' Details tab information (in a Spreadsheet) for OLD server and for NEW server, then have the DBA confirm which project Schemas are in which ORA server, you may find your issue lies in the confusion of migrating projects and accidentally taking the wrong ones offline or *worse* NOT taking them offline and using one from BOTH servers.
Good Luck!