Short summary: reboot the router
This one can have you tearing your hair out trying to debug because it appears that the access point is the problem. Testing with a variety of devices and locations reveals a strange pattern of function/non-function.
Specifics: Win 10 laptop, desktop and VM all suddenly start timing out instead of connecting while an Ubuntu VPN continues to work thru a Verizon home router.
First appearance is the Win10 patch has gone bad but reapplying does not fix it.
Take the laptop to another location with a Comcast router and the VPN connects. Tether the desktop to a phone hotspot to bypass the home router and the VPN connects. Now it looks like the router has the problem.
Wireshark monitoring of a failed connection reveals the IKE negotiation works and an encrypted tunnel is setup but the first ESP communication results in an informational ISAKMP protocol return that should not be there. This is the point of failure. There are some further notices that are not acknowledged and the connection times out.
Despite all indications that it is the specific access point causing the problem, it’s actually the EdgeRouter X that’s gone squirrely. Reboot.
Jerry's blog 