The configuration looks very much the same as the one I have been using. However, I found in http://www.juniper.net/techpubs/en_US/junos15.1/topics/task/configuration/layer-2-services-port-mirroring-named-instance-configuring.html a remark saying
"Note: You cannot configure port mirroring instances on MX80 routers. You can only configure port mirroring at the global level on MX80 routers."
This would result in modifying your configuration to
set forwarding-options port-mirroring input rate 1 set forwarding-options port-mirroring family inet output interface ge-1/1/0.899 next-hop xxx.xx.92.186 set forwarding-options port-mirroring family vpls output interface ge-1/1/0.50 set forwarding-options port-mirroring family vpls output no-filter-check
Cheers,
Carsten