Pair of 3Com 4200G switches with LACP failing if using 2 ports

[JohnMurrayUK]

We have two pairs of 4200G switches.  One pair (SAN Switches) achieves 2Gbps LACP aggregation using gbics/fibre perfectly.  The other (LAN Switches) wont.

They are not interconnected.  These are two discrete pairs of switches and the configs are the same.

For the LAN Switches that won't use LACP, I can create the group etc but the link only works as long as I shutdown one of the member interfaces (in my case 1/0/50 or 1/0/52).  It does not matter which.  Odd?

dis link-aggregation verbose tells me that I get **bleep**s ACDEF for one port, but only AC for the other and it won't pass traffic unless I disable one of the ports.

Obviously suggestions welcome.

I have checked that 1/050 on switch1 connects to 1/0/50 on switch2 and the ports are not crossed over (although I find this a strange requirement anyway).

The only/major difference is that the SAN Switches only have the one LAGG, whereas the LAN  Switches have 6 and 7 respectively.  The LAN switches are in separate sites with the only connection being these two fibre links.