"Cannot execute as the database principal because the principal "dbo"
does not exist, this type of principal cannot be impersonated, or you
do not have permission"
This seems to suggest that the database does not have an owner (or it's an owner that is different from what is expected). I don't know how you ended up with that, but perhaps the import of the backup caused it.
Anyway, you can see the current owner by executing the following SQL:
(assuming your database is named the default -- otherwise substitute your own)
On my system, this owner is 'sa'.
If you want to set the owner, you can do this with the following SQL:
exec sp_changedbowner 'sa'
(Again, substitue the database name and 'sa' as appropriate)
Of course, you should only do this if I'm right about the owner being null or some other unexpected user.