This is an issue that a friend ran into. It seem like it was hard to find the correction information so posting here just to get it out there more.
Synology Diskstation 6.2 L2TP VPN Server
Windows clients could not connection but Mac could.
The fix, shown here on Synology site its to create the registry DWORD
Computer\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\PolicyAgent\AssumeUDPEncapsulationContextOnSendRule
Set the value to 2
Reboot and should work.
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