I am able to log in fine but when the game actually tries to launch, the window goes black and stops responding.
I've tried different compatibility modes and disabling full screen optimizations. It's also being run as administrator.
Any help would be greatly appreciated!

Disregard this. Solved it.
In case anyone else comes across same issue, it was because I had remnants of a previous FFXI/POL installation that got left over after the uninstall. Was enough to confuse the boot.exe