VPN - "The L2TP connection attempt failed because security policy for the connection was not found"

Question

Connecting Windows 10 VPN to Draytek routers on numerous sites all returns the same error. I'm using PSK. Sometimes a reboot works, sometimes it doesn't. Have followed all steps in the similar message (The L2TP connection attempt failed because the security layer encountered a processing error during the initial negotiations with the remote computer), but this error of mine is "security policy not found". Where is the security policy and why does it have a problem finding it sometimes??