I could not begin to imagine why an INSERT would require "ALTER SCHEMA" as
a permission.
This looks like a lack of access to the 'Sources' table, which can
happen if the user that creates the table is missing certain
properties. I think that if the account that created the table has the
SA (Server Administrator?) role on the server then the table should be
accessible to everyone. dbo may also be required. If an unprivileged
user creates a table, I believe that, by default, it is inacessible to
other unprivilieged users.
So: verify that the table was created by SA (possibly dbo).
Disclaimer - this is all off the top of my head. I got this from the
docs, but I have a tough time getting my head around msdn navigation
so I'm not sure I could point you to where I found this information.