Supporter subscribe stuck in loop

Hello,

When a new user tries to subscribe on supporter he goes from

domain/subsite/wp-admin/supporter.php to

domain/subsite/wp-admin/supporter.php#supporter-php

or domain/subsite/wp-admin/index.php#supporter-php

over and over and he cannot subscribe

Multisite 3.1.2

Product theme

Membership plugin deactivated