Hello and thanks for your reply.
Switching -> Spanning Tree -> Spanning Tree admin mode is disabled.
The cluster consists of 8 hosts, where every host is connected with 3x RJ-45 wires to this switch. Two data paths in one VLAN, one for IRMC management in another VLAN. So, 15 ports are covered from 8 hosts, and the behavior is the same. I'd say this answers if this happens on other ports.