I just stumbled across a CONNECTX-6 for sale at a good price (<400€). I almost bought it before I noticed that it was advertised to be only compatible with a MUA9502H-2SF UFM.
The complete part number is MCX653106A-HDAT (PCIe) which is also sold without this limitation.
Can I use this card in my Linux based server or does it use a special firmware / is locked in some other way that might prevent this?
You are mixing levels or architecture and their effects, network card + sfp compatibility is isolated thing from rest of the compute system.
If card does not work with module Y, then its generally end of it. Toss Y and either buy officially supported X, or find third party oem that flashes their modules with vendor specific id like fs.com.
Restrictions on what sfp modules works are arbitrary, undocumented outside community sources and vary per manufacturer.
You might have misunderstood my concerns. I’m not asking about compatibility with specific QSFP28 modules.
I’m asking if the network card will work in my system at all.
If it really is an MCX653106A-HDAT then it should work just fine.
The other part you mentioned is a fabric manager, so it is probably talking about needing that specific Mellanox/Nvidia fabric manager to function for Infiniband.
Airflow. You need airflow Youll need to strap a fan relatively close to it in your case. Regular desktop airflow is not enough. Temps will go to 80C pretty easily.
That’s what I was thinking as well. But in today’s world you can never be sure what kind of restrictions a manufacturer has implemented in their firmware… If I don’t hear about reports that this is the case with CONNECTX-6 cards, I’m just going to have to risk buying it.
I’m well aware of that 
. It’s going to be used in my Supermicro SC219 2U server case, which has plenty of airflow.