Microsoft made a mistake in their latest patch regarding L2TP which has been rolled out (but I've yet to receive it).

Details; https://www.bleepingcomputer.com/news/m ... nnections/

Is RouterOS L2TP - configured as a server - affected by this? (Specifically it breaks if L2TP server sends "Vendor Id")

Sadly yes.

MS is talking about removing VendorID from IPSEC but I do not see the option to remove this.
Maybe someone else have found a workaround other than remove the January Patch from MS.

What I read in other places (https://borncity.com/win/2022/01/12/win ... te-kaputt/), this is not a Mikrotik issue, since report are made with other L2TP/IPsec system. It seem to be local a Windows 10/11 & L2TP/IPsec issue. So I doubt it's on MS to restore normal.

Tested with this new update and sadly I'm getting this error when trying to connect with windows 10:
The L2TP connection attempt failed because the security layer encountered a processing error during initial negotiations with the remote computer.

I just uninstalled KB5009543 and its working fine now.