P: Won't start. Ask to change permission of folders, but that doesn't solve.

Yeh, that (the heavy handedness) will be fixed in the next Lr update. Our working assumption earlier was that when you have folder permission issues like these, Lr is not going to work correctly -- crashes, hangs and weird behaviors as reported by many https://feedback.photoshop.com/photoshop_family/topics/lr-6-4-mac-os-x-10-11-3-hangs-freezes-crashes....

People aren't shooting the messenger, they're shooting the heavy-handed enforcement agent that the messenger has brought along that prevents the software from starting up.

The in-app "Learn More..." document https://helpx.adobe.com/lightroom/kb/lightroom-cc-2015-4-lightroom-6-4-crashes-on-launch.html has been updated with the latest info and workaround.

@GrAHaM Why this has anything to do with the subscription? You have an incorrect Mac OS X user account setup that could prevent Adobe software from working correctly. No one know how your machine got into that state. I have some circustantial evidence that it was due to some Mac OS X upgrade. As I have mentioned elsewhere, it is no coincidence that Apple needed to create a help doc https://support.apple.com/kb/PH21997?locale=en_US&viewlocale=en_US specifically for the El Capitan to address the customer's file permission and ownership issues.  Lr 6.5 is just the messenger that exposes such subtle issues and reports to the customer. It also offers a best effort method to fix it automatically for the customer. In most cases, it should be sufficient. But there are exceptional cases out there that need to be dealt with exceptionally. Complaining on Adobe and shooting the messenger will not address the root cause of the problem.

So updated lightroom and have same issue, i have work to process and cannot do it . This is utter rubbish, and. Not what i pay adobe a subscription for a buggy update that leaves me with no working software. Tried all above and. O joy, why should i have to make a new user account utter tosh!

Adobe Lightroom 6.5 just surfaced the potential folder permission issues on the user's Mac OS X account setup. The file system permission issues like these could trip up some Adobe applications (Lightroom's included) in some unpredictable ways (see https://feedback.photoshop.com/photoshop_family/topics/lr-6-4-mac-os-x-10-11-3-hangs-freezes-crashes...).

These account setup issues need to be addressed by the customer from the Mac OS X. For Lightroom, we would consider logging the full path of the folders that has the permission issues and possibly making the success of the permission check optional to allow one to continue working in Lightroom despite the problem (customers would be at their own risk).

Thanks Simon Chen,
Option #1 did work! The difference was the "Click the Action pop-up menu , then choose “Apply to enclosed items.” part.

Hope Adobe fix this bug quickly for others.

Could you follow Dirk's method in this thread to see if it works for you?

https://feedback.photoshop.com/photoshop_family/topics/lightroom-cc-2015-5-lightroom-6-5-camera-raw-...

And I summarize the steps here:

Option #1:

Please follow https://helpx.adobe.com/lightroom/kb/lightroom-cc-2015-4-lightroom-6-4-crashes-on-launch.html to make sure you have read/write permissions to the folders and all its subfolders, if the automatic repair in Lr 6.5 does not fix it for you. Please follow Apple’s help doc https://support.apple.com/kb/PH21997?locale=en_US&viewlocale=en_US to make sure you correct the permission on the folders AND all their enclosed items. From Apple help doc:

Apply permissions to all items in a folder or a disk

1. Select a folder or a disk, then choose File > Get Info.
   
   
2. Click the lock icon  to unlock it, then enter an administrator name and password. 
   
   
3. Click the Action pop-up menu , then choose “Apply to enclosed items.”

Option #2: If the above steps does not cure your user folder permission issue, try the following commands in the Terminal app

1. Type "Terminal" in the Spotlight search box and launch Mac OSX's Terminal app
2. At the Terminal command prompt, type exactly the following and then hit the enter key (this will remove the Access Control List on the folder and all its subfolders recursively)

chmod -RN ~/Library/Preferences/Adobe

3. Try launch Lightroom to see if it works. If it still does not work, go back to the Terminal command prompt, type exactly the following and then hit the enter key

chmod -RN ~/Library/Caches/Adobe

4. Try launch Lightroom to see if it works. If it still does not work, go back to the Terminal command prompt, type exactly the following and then hit the enter key

chmod -RN ~/Library/Application\ Support/Adobe

Option #3:

If you tried the option #1 and #2 and it still does not work, I would suggest making a backup of the user folders mentioned in https://helpx.adobe.com/lightroom/kb/lightroom-cc-2015-4-lightroom-6-4-crashes-on-launch.html and try to delete them. 

Option #4:

Setup a new user account on the machine and run Lightroom from that new account instead.

Did you make sure you clicked the cog and set "Apply to all enclosed items" too?

When you say that a backup catalog fixed it, can you tell me a bit more about that?