LACP on S4048-ON with 2x S5248F-ON not working

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 protocol

The 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...