A connection to the server has failed. (status=503)

I have changed memebership rule for a role. After that I click on "Evaluate membership rule now' and click on apply. It works when the members count is between 1 to 400.

I have changed the rule again to match 1500 users. It does does work, keeps spinning and gives A connection to the server has failed. (status=503) error on the console.
Actual users count that match the rule are 1059549 users.

select * from user_provisioning_attrs where policy_eval_needed =1;
I see zero results here.