FEX ports are intended for servers/end hosts and not any switches. If the port is going into a BPDUGuard ErrDisable state, it means it is receiving BPDUs. You can verify via:
switch# show spanning-tree interface ethernet 103/1/19 detail | i i BPDU|VLAN
Ideally, you should see all the received counters at 0. If you'd like to reset the counters and shut/no shut the port again to confirm, you can clear them via `clear spanning-tree counters interface ethernet 103/1/19` then check the show command above again to see if it has received any BPDUs since the counters were cleared.
P.S: All FEX Host Interface (HIF) ports have BPDUGuard enabled by default. If you can guarantee that there's no loop being created by the switch that is connected to a HIF, then you can configure BPDUFilter to prevent the port from going err-disabled. Just keep in mind that is against Cisco's recommendation and general best practices.
- Andrea, CCIE #56739 R&S