View Job Properties Fails for Job Owner
I had the same exact problem - I had a new user which wanted to add jobs. I added this user to msdb db and added him to sqlAgentOperatorRole and db_datareader, db_datawriter. Still, when trying to add a new job, the user was getting the same error:


  Cannot execute as the database principal because the principal "guest" does not exist, this type of principal cannot be impersonated, or you do not have permission. (Microsoft Server, Error: 15517)


I traced it down and it looks like when trying to add a new job (or doing many other things...) the following query is run:

exec master.dbo.sp_Msdbuserpriv 'serv'


So, obviously I had to add the new user to Master as well:

use master
create user [youNewUser] for login [yourNewUserLogin]
alter user [youNewUser] with default_schema=[dbo]


And now it works like charm...