Hi Romolo82,

Please compare the self IP of both the LTMs, and see the default port lockdown settings, and see what port setting is allowed on those each self IP.

Check and compare all the self IP  allows service details on both the working and non working F5.

Network section related setting including Self IP settings do not copy/sync from one box to another in HA setup, only LTM section or other config get replicated, so check if there is any config mismatch in Self IP settings. Details which are not visible from GUI. only CLI you can get these settings to compare further

list /net self

This will result in an output that looks like the following:

net self internal {
    address 10.10.10.1/24
    allow-service all
    traffic-group traffic-group-local-only
    vlan internal_vlan
}

https://my.f5.com/manage/s/article/K17333

K17333: Overview of port lockdown behavior (12.x - 17.x)

🙏