I upgraded to Lightroom CC (6.0) from 5.7.1. Photographer Cloud Service. Since completing the install, very time I go to start LR, I get an error box stating..."The action cannot be completed because the other program is busy. Chose 'Switch to" to activate the busy program and correct the problem". There are two boxes to click on, "Switch to", and "retry". Clicking on either will start LR but it takes several clicks to get it complete the start up.
I read a "fix" on the lightroom forum that deleting the SL Cache folder would fix it. That did for a couple days. However that error box has returned when I went to use LR this morning, the day after 6.1 installed. LR 6.1 did start normally after several cold boots of the computer the day the update was installed.
This is on a windows 7-64 bit desktop with 3.4 Intel 7 chip, 18 gig of memory, over 1 TB of free space on the drive LR is loaded on.
For me, this was caused by some of my additional hard drives spinning down because of my power settings. Changing the power settings to never turn off hard drives fixed the problem.