Is your service broker target queue in msdb? The user running the activation procedure will not be impersonated across databases, so if your service broker objects are in Database1, then the activation procedure cannot execute procedures in other databases (like msdb).

I found an article explaining this a little bit better :

http://www.databasejournal.com/features/mssql/article.php/3800181/Security-Context-of-Service-Broker-Internal-Activation.htm