Recently hit an issue between a Universal 5420 switch running EXOS in a stack connected to VSP 7432 switches. Normal practice to use different slots for the uplinks ie 1:52 and 2:52 in a LAG. I noticed 2:52 was not coming up.
On slot 1, the Master switch, Forward Error Correction was ON and this matched FEC settings on the VSP 7432 interfaces (100G channelized to 4 x 25 with breakout cable).
The default for EXOS should be that FEC is disabled. Disabling FEC on the VSP 7432 side brought the link up on 2:52. Disabled FEC on 1:52 allowed all ports to be active.
Bottom line, FEC must match both ends for the link to come up. I have raised a ticket as to why slot 1 would enable FEC on its ports but not on backup or standby switch ports.