PVE 9.1 upgrade: Connection error 596: Connection timed out

[kiwistagnz]

After upgrading a cluster to PVE9.1 from 9.0, it seems to have broken my login to any of the nodes.
It seems I can briefly log in post node boot, but soon after I cannot.
I have seen the same issue some time ago with older PVE versions in older forum threads, however haven't had any luck with any suggestions.

Servers:
Mix of 4x HPE dl380 G7 and DL360 G7 servers.
Primary network interface is 4x ethernet ports in LACP
Cluster interface is SFP+ connection.
Switch stack is Allied Telesis x510.

Servers all run CEPH in a balanced layout.

Only using IPv4.

Login GUI shows:

Login failed:
Connection error 596: Connection timed out
Please try again

Any suggestions as to how to dig deeper would be appreciated.

 

[kiwistagnz]

Status data from cluster config:

$pvecm status

Cluster information
-------------------
Name:             SiX
Config Version:   8
Transport:        knet
Secure auth:      on

Quorum information
------------------
Date:             Thu Dec 11 20:48:52 2025
Quorum provider:  corosync_votequorum
Nodes:            4
Node ID:          0x00000002
Ring ID:          1.438
Quorate:          Yes

Votequorum information
----------------------
Expected votes:   4
Highest expected: 4
Total votes:      4
Quorum:           3
Flags:            Quorate

Membership information
----------------------
    Nodeid      Votes Name
0x00000001          1 192.168.19.13
0x00000002          1 192.168.19.11 (local)
0x00000003          1 192.168.19.12
0x00000004          1 192.168.19.10