So, back I am.
Unfortunately I was not able to duplicate the conditions that caused the first error ("missing dbo schema"), because I had to configure my SQLMonitor Hosted acct with a different server.
But I am still facing some errors that are preventing the monitoring, and in my understanding they should only generate alerts. This happens when I have a database marked as Read-Only (or a Snapshot) and the login used to collect the data is not given SA rights.
In this scenario, the monitor will try to connect to the unavailable database, fail, and stop monitoring. I believe it should only fail the connection and generate an alert, continuing monitoring the other databases.
"Configuration > Monitored servers > Show Log (for a specific instance)"
Date: 5 May 2014 4:46 PM
Outcome: Incorrect credentials or insufficient permissions
Exception message: The server principal "<my_login>" is not able to access the database "<my_snapshot>" under the current security context.
The only workaround I found was to mark the database as writable, create the monitoring login, and mark it as read-only again - or just drop the snapshot in this case.