Shouldn't the policy send the username column like this?
CREATE SECURITY POLICY rls_CustomerFilter
ADD FILTER PREDICATE dbo.rls_customer_check(username)
How to post data/code on a forum to get the best help: Option 1 / Option 2
Are you seriously taking the advice and code from someone from the internet without testing it? Do you at least understand it? Or can it easily kill your server?