Can't get multicast to work (iperf)

Question

I have some problems to get SLP (service location protocol; I play around with the OpenSLP implementation) to work. It uses multicast on 239.255.255.253, port 427. To narrow down the problem I decided to test the multicast connection with iperf first (without any SLP related stuff).

So on one computer I set up a server using

iperf -s -u -B 239.255.255.253 -p 427 -i 1

And on the other computer I try to send some data using

iperf -c 239.255.255.253 -p 427 -u -T 32 -t 3 -i 1

But the server doesn't receive any data.

The output on the server side is:

bind failed: Cannot assign requested address
------------------------------------------------------------
Server listening on UDP port 427
Binding to local address 239.255.255.253
Joining multicast group  239.255.255.253
Receiving 1470 byte datagrams
UDP buffer size: 64.0 KByte (default)
------------------------------------------------------------

On on the client side:

------------------------------------------------------------
Client connecting to 239.255.255.253, UDP port 427
Sending 1470 byte datagrams
Setting multicast TTL to 32
UDP buffer size: 64.0 KByte (default)
------------------------------------------------------------
[  3] local 152.96.201.188 port 53777 connected with 239.255.255.253 port 427
[ ID] Interval       Transfer     Bandwidth
[  3]  0.0- 1.0 sec   129 KBytes  1.06 Mbits/sec
[  3]  1.0- 2.0 sec   128 KBytes  1.05 Mbits/sec
[  3]  2.0- 3.0 sec   128 KBytes  1.05 Mbits/sec
[  3]  0.0- 3.0 sec   386 KBytes  1.05 Mbits/sec
[  3] Sent 269 datagrams

So the problem seems to be on the server side because it can't bind the multicast address. What could be the problem there? How could I debug it?

Btw, it looks like the server can successfully join the multicast group address. Because when I try the same command in a private network without a router (just an unmanaged switch), it says (in addition):

multicast join failed: Invalid argument

Both computers run Win 7 and are connected to the company network. They are inside the same VLAN (ping is successful). IT department says that multicasting is enabled in this subnet.

Hints on how I could debug this would be much appreciated!