Installed Lightroom 6 on a Windows 8.1 system. Provided a serial # but it complained that I need Internet access. Internet was working fine so I selected Connect Later. Did this at least 8 times. Now when I try to run, the icon shows up on the taskbar but is stalled there. When I hover over the icon it shows a thumbnail of the screen to create a new catalog, but it will never expand the screen. The only option I have is to close the tiny window with a right-click. Then I start over and it is the same - no possibility to start Lightroom even after a reboot and then a re-install! Can't even get back to the bogus attempt at validation of my serial number.
Read here how to correctly set up a standalone licensing for LR6: https://helpx.adobe.com/lightroom/kb/serialize-lightroom-cc-trial-to-activate-as-lightroom -6.html#
If you have a subscription, you simply need to logon to CC using the CC desktop application.
Also make sure to have it updated to 6.0.1.
I decided on a variation of your suggestion. I uninstalled and then download the latest CC version from Adobe. However the same problem persisted. So it was now time for some lateral thinking! Since the startup was not even getting to the Create Catalog screen, I figured something was wrong there. So I grabbed a laptop, installed Lightroom and then copied the Pictures\Lightroom folder over to my desktop PC. It worked! Now Lightroom is running fine. All I have to do is enter my serial number and there is no looking back