By default, Acronis Management Server Service is running under "ams" user. This user is created during installation unless you specify another user during installation.
If you want to change this user manually after installation, make sure that the user you specify for Acronis Management Server Service is a member of Acronis ApiGateway Users group. This is required for successful communication between Acronis Management Server child services.
If you need help adding a user to a group, see this manual.