SQL Server EXECUTE AS trouble
This is explained in Extending Database Impersonation by Using EXECUTE AS. The EXECUTE AS context is trusted only in the current database and allowing it to spill over to other databases is a escalation of privilege attack vector.

There are two solutions, both described in the article linked above:


the easy one is to mark the database TRUSTWORTHY: ALTER DATABASE [source_db] SET TRUSTWORTHY ON;. Although easy, is also dangerous in as it makes the dbo of source_db a de-facto sysadmin.
the safe one is use code signing, see Call a procedure in another database for an example. This is more complex, but is 100% buletproff security.