Broken ECMP ipv6 with SRX1500 in paketmode

hi,

I have here a setup with srx1500 in paketmode. If I try to enable ECMP with ipv6 all runs finex except Clients that runs Windows Server > 2012.  I found out that Windows Server Systems have ECN default enabled since Server 2012.

If I disable ECN on the Windows Client all runs fine. Linux Clients with ECN enabled run fine also.

without ECN:

SYN:                       Client – Router – > SERVER A
SYN ACK:             SERVER A – Router –> Client
ACK:                      Client – Router –> SERVER A

With ECN on Windows > 2012

SYN:                       Client – Router – > SERVER A
SYN ACK:             SERVER A – Router –> Client
ACK:                      Client – Router –> SERVER B

Does anyone have any sort of idea of whats wrong or how to prevent this?