I am running into the exact same issue with a fresh install of the AdminUI 12.8 on Windows Srv 2012 R2. I have run this with debug logging started and see the following.
I let it sit for about 30 minutes and then finally hit the Shift button a few times on the keyboard and magically it finished the install!
The installer used was ca-adminui-12.8-win64.exe.
The build is 12.80.0.1719.
I tried to reinstall this 4 times prior to running it in DEBUG log mode and waiting and waiting and then hitting Shift somehow magically got the installer to finish after 30 minutes. I noticed when looking at the task manager during this step the Java Virtual Machine doing the installer was running extremely high CPU and Memory % spiking to 95 to 100 % often.
Hope this helps!
Adam Rusniak