Just to finish this topic: I don't exactly know why the configuration suddenly worked, but it does now and it is stable. Maybe the switches just needed a small kick...
We got two new S5248F-ON which I configured with VLT. I connected lots of compute nodes with LACP to the new switches, all working fine. But I've been trying all day to connect our S4048-ON using 2x 40GBit cables and cannot get this to work. At first I thought it was STP interfering, but even with STP disabled, no LACP. The 5248s report all ok:EthSW25G-1# show interface port-channel 55 summary  LAG     Mode      Status    Uptime              Ports 55      L2-HYBRID up        01:18:35            Eth 1/1/55:1 (Up) But the 4048 shows no ports:EthSW10G(conf)#do show interfaces port-channel 55 brief     LAG  Mode  Status       Uptime      Ports           L   55   L2    down         00:00:00    But these ports  are configured:interface fortyGigE 1/50 no ip address !   port-channel-protocol LACP   port-channel 55 mode passive  no shutdown ! interface fortyGigE 1/51 no ip address !   port-channel-protocol LACP   port-channel 55 mode passive  no shutdown !I tried active and passive, no change. All physical interfaces are no shut, lines are up: EthSW10G(conf)#do show interfaces fortyGigE 1/50 fortyGigE 1/50 is up, line protocol is up EthSW10G(conf)#do show interfaces fortyGigE 1/51 fortyGigE 1/51 is up, line protocol is up But the LAG is down:EthSW10G(conf)#do show interfaces port-channel 55 Port-channel 55 is up, line protocol is down(minimum links not up) Created by LACP protocolThe 5248 have the latest OS10 10.5.3.2 installed, I updated the 4048 to OS9 9.14(2.11) today. What am I missing? Is LACP not possible in this setup?  BTW: When I configure the switches to not use LACP, so just use the cables directly, it works fine. I am not sure if this is a good setup for production though (if one link goes down: packet loss) I'd be really grateful for any kind of hint why this is not working...  
LACP on S4048-ON with 2x S5248F-ON not working