Thunderbolt 2 dock at iMac 12.1 -> No Displays

Dear Fedora community, I’ve got an iMac 12.1 from 2011 and two Thunderbold 2 docks here. One from Belkin and one from elgato. I was trying to connect one of them to the iMac, since I wanted to have USB 3.0 and at least one external display connected to the iMac. The docks both seem to be working in general, since the USB ports and the additional network adapter show up and can be used. But plugging in an external monitor via Displayport/Thunderbold or HDMI has no effect.

Once e.g. the Belkin dock is plugged in, a Thunderbold 2 bridge is showing up:

09:00.0 PCI bridge: Intel Corporation DSL5520 Thunderbolt 2 Bridge [Falcon Ridge 4C 2013]
0a:00.0 PCI bridge: Intel Corporation DSL5520 Thunderbolt 2 Bridge [Falcon Ridge 4C 2013]
0a:01.0 PCI bridge: Intel Corporation DSL5520 Thunderbolt 2 Bridge [Falcon Ridge 4C 2013]
0a:04.0 PCI bridge: Intel Corporation DSL5520 Thunderbolt 2 Bridge [Falcon Ridge 4C 2013]
0a:05.0 PCI bridge: Intel Corporation DSL5520 Thunderbolt 2 Bridge [Falcon Ridge 4C 2013]
0b:00.0 USB controller: Fresco Logic FL1100 USB 3.0 Host Controller (rev 10)
0c:00.0 Ethernet controller: Intel Corporation I210 Gigabit Network Connection (rev 03)

So I guess the Thunderbold ports from the dock should be working. The dock itself is of course also showing up in boltctrl and is authorized:

 ● Belkin International, Inc. Thunderbolt 2 Express Dock HD
   ├─ type:          peripheral
   ├─ name:          Thunderbolt 2 Express Dock HD
   ├─ vendor:        Belkin International, Inc.
   ├─ uuid:          00000000-0000-0008-80e6-b70b56e4b851
   ├─ generation:    Thunderbolt 2
   ├─ status:        authorized
   │  ├─ domain:     6088111e-0a00-0100-ffff-ffffffffffff
   │  ├─ rx speed:   10 Gb/s = 1 lanes * 10 Gb/s
   │  ├─ tx speed:   10 Gb/s = 1 lanes * 10 Gb/s
   │  └─ authflags:  none
   ├─ authorized:    Sun 15 Aug 2021 19:58:21 UTC
   ├─ connected:     Sun 15 Aug 2021 19:58:21 UTC
   └─ stored:        Thu 05 Aug 2021 13:10:09 UTC
      ├─ policy:     auto
      └─ key:        no

Anybody with an idea? Thanks!