We it had a test environment with SP 20. We have unistalling the SP 20 (without upgrade the Database) and we have installed the SP 18. When upgrading the Database we have obtained the next error. (View attach). Also I attach the logdatabase file.
Although the Service Packs are pretty good at UP-grading databases you cannot really expect them to DOWN-grade? you'd need to restore a dump of the SP18 DB taken before you went up to SP20?
But oddly the messages in your logs seem to refer to "wrong username for repository tables" are you sure the DB connection was defined with the correct 2 names (possible on Oracle) or one name (MS SQL and optional on Oracle)?
If during the upgrade from SP18 to SP20 you upgraded the DB (as you should or else the server wouldn't work), you cannot rollback no matter what.
Even if you try to "upgrade" SP20 to SP18 it will lead to problems. Your only option is to find a DB backup of SP18 and restore it, or import an image of the production (I assume that production is SP18)
this doesn't help with getting back to SP 18 but bear in mind that when you uninstall OVSD some of the folders and files are still left on the system. I also delete or rename the folders in the /hewlett-packard/servicedesk folder structure and delete any client folders to ensure a clean application uninstall and good reinstall