After some tracing I've found that MT incorrectly rejects LCP Code 12 packets. They are recognized as rejects of some random code. If this code is essential (9 for example) Windows drops the connection with 732 error.
Here's how it happens:
[1656] 12:54:12:315: <Protocol = LCP, Type = Identification, Length = 0x14, Id = 0x2, Port = 3
[1656] 12:54:12:315: <C0 21 0C 02 00 12 09 74 17 D8 4D 53 52 41 53 56 |.!.....t..MSRASV|
[1656] 12:54:12:315: <35 2E 31 30 00 00 00 00 00 00 00 00 00 00 00 00 |5.10............|
[1656] 12:54:12:315: >PPP packet received at 07/30/2004 09:54:12:315
[1656] 12:54:12:315: >Protocol = LCP, Type = Code-Reject, Length = 0x14, Id = 0x3, Port = 3
[1656] 12:54:12:315: >C0 21 07 03 00 12 09 74 17 D8 4D 53 52 41 53 56 |.!.....t..MSRASV|
[1656] 12:54:12:315: >35 2E 31 30 00 00 00 00 00 00 00 00 00 00 00 00 |5.10............|
[1656] 12:54:12:315: PPP Code Reject rcvd, rejected Code = 9
[1656] 12:54:12:315: Unpermitted code reject rcvd. on port 3
Code reject packet from MT seems not to conform to rfc1661
conforming packet should look more like C0 21 07 03 00 12 0C 02 00 12 09 74 17 D8 4D 53 52 41 53 56
"The Rejected-Packet field contains a copy of the LCP packet which is being rejected. It begins with the Information field" says rfc1661 but MT begins with LCP DATA field.
So it's MT bug. I hope it'll be fixed soon