Just to note,

I was recently having a similar issue. The test database I was working on gets restored from back up periodically from another machine. So that breaks the Sql Server user on the database mapping to the login on server. Thus I need to re-create the test user account (that I employ on the database ) to match to that of the login on this test server. As an additional step, since I call stored procedure in my reports, I also have to grant the execute permission to that user in that database. So depending on your circumstance you may need to allow access to views and tables if the account you use to connect with in ssrs does not already have read access on the database. Just thought I'd share.