It has been a while since I've been working on this issue (had to finish my degree first), so I guess it's time for an update:

From version 90 all was working fine again and I am currently running version 97 (latest).
I've been able to reproduce the errors. What appears to be the case?

Any Windows server that has a primary NIC in my LAN and is a domain member, fails when connecting through a dedicated NIC in the iSCSI LAN (or VLAN, in case of a virtual machine).
Contrary, any server that has its primary NIC in another LAN (or VLAN) and subnet, succesfully authenticates, all settings on the DSS being equal.

Now I'm getting really puzzled...