What you see in the Develop module?
Now I have both versions installed on my PC (Lightroom 5 and Lightroom CC), but I'm only able to access full feature of 5, not CC. The "Develop" view of CC is not accessible.
This behavior indicates that it thinks you do not have a creative cloud license. Did you actually get a CC subscription? If you did and Lightroom does not work right, you should probably log out of the creative cloud app (gear icon->preferences) and log back in. If it still doesn't work, you probably should get help from Adobe support directly. This forum is mostly other users, not Adobe folks. They have an online chat support system that works well at least it has for me when I had similar issues with my CC subscription.
In Lightroom CC, in the Develop module I can see a blue rectangle, with two
white diagonals forming an X.
Edit >> Preferences >> Performance
Then disable GPU acceleration.
You should go to the website of the manufacturer of your graphics processor and check for driver updates. After updating you could try switching GPU acceleration on again, otherwise leave it disabled.
Thank you! This helped: now I can see the image in the Develop module.
But it still complains that "The folder could not be found", and the
controls are grayed out.
Go back to the Library and switch to another folder. Then click back on the folder you need.
Great, it's working fine now.
Thank you for the advice.