Synology CS 407e: altboot fails to load installer.

I'm trying to install netbsd, latest binary snapshot 201105062100Z on
a Synology Cubestation 407e.
I uploaded altboot.bin with kermit over serial and started it without problems.
I set up a dhcp server and a tftp server, both are confirmed working
since the cubestation gets it's correct IP when I let it load
normally.
TFTP server was tested by connecting another computer and getting the
netbsd-INSTALL file.

I loaded wireshark on my server to see if any dhcp requests are sent.

Now to the problem:

I run altboot by loading it from memory with "go 1000000
tftp:netbsd-INSTALL" as in the instructions.

Altboot loads, but never requests any IP from my server, doesnt seem
to use ethernet at all, then gives the messages:

xmit failed
xmit failed
xmit failed
xmit failed
bootp: no reply
"" not found
load failed. Restarting...


Any idea what's going on?

/Oscar