I came across the error below while installing Exchange 2010 SP1 today.
I checked my system mailbox (federated one) and it was pointing to a non-existent database. I tweaked the account setting to point to the right database and Exchange setup was happy.
And how did you do all of that? I have a exchange2010SP2 rollup 4 enviroment and all I am trying to do is intall more cas and hub servers and I get the error above.