L2TP/IPsec VPN from Windows 10 fails with error "remote server is no responding..."

Background

• Synology DS413 NAS configured as L2TP/IPSec VPN server and located behind Draytek Vigor 2860 NAT.
• L2TP VPN pass through enabled on Draytek Vigor Router (VPN and Remote Access > Remote Access Control > remove checkboxes from Enable IPSec VPN Service and Enable L2TP VPN Service).
• UDP ports 1701, 500 or 4500 are forwarded from Draytek Router to Synology NAS.
• Windows 10 Pro client trying to connect using built-in Windows VPN client.
• VPN connection over PPTP works fine.

Issue

• VPN connection over L2TP/IPsec fails with following error:
  
  The network connection between your computer and the VPN server could not be established because the remote server is not responding. This could be because one of the network devices (e.g, firewalls, NAT, routers, etc) between your computer and the remote server is not configured to allow VPN connections. Please contact your Administrator or your service provider to determine which device may be causing the problem.

Windows Defender - The threat service has stopped. Restart it now

Issue

Windows 10 Security Centre shows red X next to Virus & thread protection. Status reads "The threat service has stopped. Restart it now."

Clicking Restart Now button doesn't help. Following error is displayed "Unexpected error. Sorry, we ran into a problem. Please try again."

Windows Firewall doesn't save any changes - keeps reverting to defaults

Issue

All changes to Windows Firewall (application, port rules, etc.) stop working and disappear from UI few seconds after they were added. The same happens if Windows Firewall is disabled completely - few seconds later it gets re-enabled.

Following Info level event are constantly getting logged in Windows Applications and Services Logs:

Event ID: 2032
Log Name: Microsoft / Windows / Windows Firewall With Advanced Security / Firewall
Source: Windows Firewall With Advanced Security
Description: 
Windows Firewall has been reset to its default configuration.
ModifyingUser: SYSTEM
ModifyingApplication: C:\Windows\SysWOW64\netsh.exe

Send Automatic Replies (Out of Office) to external recipients only

Outlook / Exchange Out of Office Auto-Reply system allows to send automatic replies to either internal senders only (Inside My Organization) or internal and external senders (Outside My Organization). There is no option to send auto responses to external senders only.

As a workaround you can use mailbox rules to replicate this missing functionality.