Linux 2.4.5: networking problems with bintec router

! This problem occurs, nomatter wether ECN is enabled or disabled !:

Since upgrading to Linux 2.4.5 (ECN disabled!) I'm unable to connect to any hosts which are located behind a Bintec Brick router (the brick performs port forwarding).
When I'm trying to connect from a Linux 2.2 machine, I have no problems at all.

Are there any known networking problems between Linux 2.4 and Bintec routers?