How to Fix the L2TP Connection Attempt Failed because the security layer

How to fix the L2TP connection attempt failed because the security layer encountered a processing error during initial negotiation with the remote computer.

[MegaDroidDude]

Mikrotik's L2TP-IPSec works just fine on fresh Windows 10, but the minute I touch XBox networking this above error pops up -- generic error 789 in the windows error log. There may be a conflict with the teredo driver, which reinstallation and even uninstalling doesn't fix. I keep a Window's restore point to recover from any configurations that break L2TP-IPsec.