L2TP Error connecting Windows 10 to USG 100 - all of a sudden

ORCA Posts: 2
Hello to the community,
since yesterday there is an L2TP issue connecting Windows 10 Pro (21H2) with Windows native software to Zyxel USG 100.

USG 100 behind a FritzBox 5490. Port forwarding UDP 500 & 4500, TCP 1701, ESP. FritzBox is up to date (7.29) and restarted twice. USG 100 is up to date Firmware V5.20 (ABUH.0). Restarted USG 100 twice.
Just in case Port forwarding had been corrupted, all Ports in forwarding center FritzBox 5490 have been deleted by us and configured from scratch.
Connection Method: L2TP/IPsec with preinstalled key
Using Windows VPN built-in method.

The return message from Windows 10 VPN software is:
"The L2TP connection attempt failed because a processing error occurred during the initial security negotiation with the remote computer".

This issue occurs to 30 client computers in our organization. The setup has been working perfectly for about 10 month - until yesterday!

Anyone same issue recently or any idea?

Thanx much,


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