So, it seems to be OK, but when I insert the last switch stack (fourth), my network is going down.
Four switch stacks are inserted in 19,20,21,22 master router ports
Three switch stacks are inserted in 19,20,21 spare router ports.
Forks fine until I insert fourth switch stack in the spare router.
Router ports are configured to Trunk, no native vlan, drop untagged traffic.
Switch ports are configured with the RSTP enabled and STP guard disabled.