404 after switching on WP mu

Hi,

When I try to use “user switching” on a child site of a WP mu, I have a 404 error
eg: https://ssk-formation.com/wp-content/uploads/sites/48/2017/08/Capture-d’e?cran-2017-08-25-a?-08.30.40.png

(I’m super admin r?le)

Must I do something special for use the plugin on a WP MU ?

thanks a lots
Fred