RHEL 7 High Availability node unable to join cluster, corosync reports "[QB ] Denied connection, is not ready", and reports continuous "Retransmit List"

Issue

• A cluster which was previously working for long time, suddenly starts missbehaving
• I have several nodes in the cluster that are able to join and communicate just fine, but one reports the others as "unclean" in pcs status output, and it shows constant "[TOTEM] Retransmit List" messages and "[QB ] Denied connection, is not ready" messages over and over in the logs

Sep 15 22:34:55 node4 corosync[56061]:  [TOTEM ] adding new UDPU member {192.10.1.1}
Sep 15 22:34:55 node4 corosync[56061]:  [TOTEM ] adding new UDPU member {192.10.255.254}
Sep 15 22:34:55 node4 corosync[56061]:  [TOTEM ] adding new UDPU member {192.10.255.253}
Sep 15 22:34:55 node4 corosync[56061]:  [TOTEM ] adding new UDPU member {192.10.255.252}
Sep 15 22:34:55 node4 corosync[56061]:  [TOTEM ] A new membership (192.10.255.252:4) was formed. Members joined: 4
Sep 15 22:34:55 node4 corosync[56061]:  [VOTEQ ] Waiting for all cluster members. Current votes: 1 expected_votes: 6
Sep 15 22:34:55 node4 corosync[56061]:  [TOTEM ] A new membership (192.10.1.1:12) was formed. Members joined: 1 5 6 3 2
Sep 15 22:34:55 node4 corosync[56061]:  [TOTEM ] Retransmit List: 3 4 5 
Sep 15 22:34:55 node4 corosync[56061]:  [TOTEM ] Retransmit List: 3 4 5 
Sep 15 22:34:55 node4 corosync[56061]:  [TOTEM ] Retransmit List: 3 4 5 a 
Sep 15 22:34:55 node4 corosync[56061]:  [TOTEM ] Retransmit List: 3 4 5 a 
Sep 15 22:34:55 node4 corosync[56061]:  [TOTEM ] Retransmit List: 3 4 5 a 
Sep 15 22:34:55 node4 corosync[56061]:  [TOTEM ] Retransmit List: 3 4 5 a 
Sep 15 22:34:55 node4 corosync[56061]:  [TOTEM ] Retransmit List: 3 4 5 a 
[...]
Sep 15 22:34:55 node4 corosync[56061]:  [TOTEM ] Retransmit List: 3 4 5 a 
Sep 15 22:34:55 node4 corosync[56061]:  [QB    ] Denied connection, is not ready (56061-56065-18)
Sep 15 22:34:55 node4 corosync[56061]:  [TOTEM ] Retransmit List: 3 4 5 a 
Sep 15 22:34:55 node4 corosync[56061]:  [QB    ] Denied connection, is not ready (56061-56067-19)
Sep 15 22:34:56 node4 corosync[56061]:  [TOTEM ] Retransmit List: 3 4 5 a 
Sep 15 22:34:56 node4 corosync[56061]:  [QB    ] Denied connection, is not ready (56061-56069-20)
Sep 15 22:34:56 node4 corosync[56061]:  [QB    ] Denied connection, is not ready (56061-56071-21)