At what point in the process is DISM failing?
Can you check the DISM log on your core server located at C:\Windows\Logs\DISM\dism.log? They may provide insight into what part of the DISM process is failing.
From our core server on 9.5 SP2: I am able to load the GUI from the "Manage the drivers in the Windows PE image" link in OSD. I verified that dism.exe launched as a process in Task Manager on the core, then I added a dummy driver and finished the process successfully.