We updated our Vigor3912 to firmware 4.4.3.2 in January 2025…

Shortly afterwards, we noticed strange behaviour with certain software and websites running from or accessed from, the client machines on the LAN behind the 3912. 
The same client machines would not exhibit the same behaviour when not on the network behind the 3912.

We initially assumed the issue to be with Windows 11 24h2, or one of the many ancillary patches that pulled in, which we’d also recently updated all of the client machines to.  But after rolling back a couple of machines to Windows 11 23h2, found that not to be the case.  We even looked at the locally installed anti-virus/firewall solutions as a culprit, but found them not to be the cause either.

The specific issues were –
1.      For a specialist Actuarial Tool, the tool would take circa 3 to 5 Minutes to load, once loaded no other issues found.
2.      Endpoint Patch Management/Remote Access tool – all endpoints became inaccessible, when behind the 3912, even from another machine also behind the 3912.
3.      The Office 365 Configuration Management Webpage ( https://config.office.com ), became inaccessible – timed out on connection attempts.
4.      Various other websites, (all https based) – timed out on connection attempts, but not all https based websites exhibited this behaviour.

We ultimately rolled back the firmware on the 3912 to 4.3.6.2 and all of these issues vanished. 

Looking further into the affected software and websites all failed with access timeouts, whilst performing SSL based verification/logon's to resources on the internet, via the standard port 443.