vSZ-H Mesh Not Working

[GreensTheorem]

I was testing a deployment of Virtual SmartZone High Scale 6.1.2.0.441 with a bunch of R850 APs and tried to connect one AP via Mesh.
Sadly, I wasn't able to get this AP to work. When I tried to connect the R850 AP via Mesh, it would stay at "Offline" and start Broadcasting the RescueME SSID eventually.

General Setup: vSZ-H with 3 Interfaces. vSZ Control interfaces are 10.2.1.1/16, 10.2.1.2/16, 10.2.1.3/16.
10.2.0.0/16 is located in VLAN 2. VLAN 2 has a DHCP server.

All APs except the one that is supposed to be connected via Mesh were set to "Mesh Mode: Root". 

First Attempt: 

APs have Management VLAN 2 set, Ports on Switch (ICX7650-48ZP) where APs are connected are pure trunk ports and AP has to do the VLAN Tagging for Management

1. Onboard AP via cable connection and wait until it's fully upgraded and configed, set AP Mesh Mode to Mesh AP only, wait until configed and online. Remove AP cable connection and power via PoE injector -> AP will not connect, will never show as online

(did not try Zero Touch Mesh)

Second Attempt: I thought the management VLAN is an issue, so I removed the management VLAN and configured the switch ports as dual-mode (access VLAN 2 and trunk for Client VLANs).

1. Onboard AP via cable connection and wait until it's fully upgraded and configed, set AP Mesh Mode to Mesh AP only, wait until configed and online. Remove AP cable connection and power via PoE injector -> AP will not connect, will never show as online

2. Onboard AP via Zero Touch Mesh: AP will show in "Mesh" View Mode -> Approve There. Also Approve AP in the Group View. However, AP will never show as Online, only as offline.

I tried to do some troubleshooting, but I couldn't get any meaningful info. I plugged in a cable into the Mesh AP and sshed into it. It recognised the neighbours when I executed `get mesh` and also seemed to have the correct SSID etc..

Any ideas how to troubleshoot this?