same issue on my notebook - i did a uninstall and used the adobe cleanup tool... then reinstall.... some as before... very frustrating...
on my wacom win8.1 tablet - the update went smooth - and LR CC starts without any issues....
After installing lightroom CC you need to log out from the Creative Cloud and once again Sign in.
Do this by:
Preferences>Account>Log out from the Creative Cloud.
Let me know if worked.
It's already known since yesterday that the sign out, sign in solution doesn't work for many users. This will probably require a software update to Lr CC 2015 or to the Creative Cloud from Adobe.
Thank you a simple fix, should be a first in adobe results. fixed problem in a few seconds after spending most of the afternoon searching for an answer on adobe support.