Policy error returned on task approval

Hello,

I've an ARS farm with 2 servers, SERVER1 and SERVER2, they are accessed by an alias prd-ars.domain.com 

I've set approval by primary owner on one group and modified the policy (Approval by Primary Owner (Manager)) to send notifications with the alias in the URL.

When someone add an object to the group, we receive the notification with the approval pointing to the URL with the alias, we can display it on ARS website but when we click on approve we get "Policy error".

When I change the policy to have SERVER1 in the link instead of the alias, it works.

When I change the policy to have SERVER2 in the link I get back "Policy error".

Are workflows processed only by one server?

Both servers share the same database.

Regards,

Nicolas