The issue is fixed by executing the following query on the OTRS database in effect changing the data_value text encoding to binary (it basically behaves as a blob field then).
After upgrading from OTRS 3.1.x to OTRS 3.2.2 Agent logins work but the Customer Frontend doesn't allow session creation anymore: [Thu Feb 21 11:31:16 2013][Error][Kernel::System::AuthSession::DB::CreateSessionID][333] Incorrect string value: '\x80\x00\x00\x00\x08C...' for column 'data_value' at row...