Answering myself:
Firmware for Volataire 500 EX-D can be found here: Firmware for ConnectX® - Mellanox Technologies. Look for MHEH28-XTC (PSID: MT_04A0110001)
The problem with "Modify QP error" and osm.log flooding persists even after upgrading to firmware 2.9.100 and seems related to OpenSM when using dual port end-to-end connections with two cables.
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