So this might be a similar issue to https://community.extremenetworks.com/extreme/topics/c5_b5_f_w_6_71_03_0025_issuing_unsupported_plug...
but we're running the latest firmware.
We're running c5-series_06.81.01.0027 on two C5G124-48P2's.. the modules work fine when unstacked, but when stacked, we get the unsupported pluggable module error upon boot. When we view the port status it says "Combo RJ45/Invalid/PoE".
Edit: I've tried this with both 10GB-LRM-SFPP and 10GB-LR-SFPP modules.