Hi,
you should not need to run installslave after update. Just starting the existing "Deployment Monitoring Agent" on the database server should be enough. While starting first time after update on DB Server you should get a message similar to "updating common components".
For the rest of your issue I recommend opening a case. (I assume you tried to reconfigure backup whikle running Deployment Montitoring Agent on IMC Server with administrative rights. This is state in update documentation.)
Regards, Jö