Хотелось бы не просто видеть в логах ошибки типа PPTP:error=-112, PPTP:error=-299, но и понимать что они означают.

Посещяйте раздел FAQ:
http://www.dlink.ru/technical/faq_internet.php#14

5. PPTP:error= ERROR_NO
An error occurs on PPTP control connection. Currently, there are the following errors:
Error Cause
-111 PPTP server is unreachable. Check configuration!
-112, -115 Fail to establish connection to PPTP server
-200, -214, -299 PPTP control connection was aborted abnormally
-301, -302 PPTP server rejects connection request
-312 Command channel not established – general error
-313 Command channel already exists
-314 Requester is not authorized to establish a command channel
-315 The protocol version of the requester is not supported
-303, -304 PPTP server rejects outgoing call request
-332 Outgoing call not established – general error
-333 Outgoing call failed due to no carrier detected
-334 Outgoing call failed due to detection of a busy signal
-335 Outgoing call failed due to lack of a dial tone
-336 Outgoing call was not established within time allotted by PAC
-337 Outgoing call administratively prohibited
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А сопоставить с этим списком можно как то?
Ошибки VPN Microsoft

Устройство не работает под управлением ms win, так что сопоставить нельзя.