Lightroom Desktop: Can’t start app: fatal database error/database malformed

One last thing to try, Betty before we start requesting files from you. Have you attempted a preference file reset?

A Preference File will survive a Lightroom uninstall/reinstall. Sometimes weird behavior is corrected/cured by resetting the preferences. 

Reset Procedure:

1. Close Lightroom.
2. Hold down [Alt/Opt]+[Shift] while restarting Lightroom. 
3. Overwrite the Preferences when prompted by the dialog.
4. Close Lightroom.
5. Restart Lightroom.

Does the behavior continue after resetting the preferences?

If this doesn't cure the issue, we will start getting diagnostic logs from you. 

Thank you for your patience. 

thank you Rick and yes, I did try a preference file reset by following your instructions above and that didn't help either...I still have the same dialog box.  I'm not sure what is happening here but below is the dialog box that appears

And to confirm,

• You've closed Lightroom
• Right-clicked on Lightroom Library.lrlibrary and chosen "Show Package Contents"
• Located the MCAT file in the folder and renamed it to Managed Catalog.old
• Restarted your computer
• Launched Lightroom

And still have the error?

yes.  I did that also.  I also uninstalled and reinstalled LR seeing if that would do the trick but it didn't.  Then I tried the Preference reset that you also suggested...still the same screen shot that you see above when I click on Lightroom CC - desktop.

Can you give me a screenshot of Finder after the "Show Package Contents" step?

thank you Rikk - here you go...as you can see, I renamed the managed cat file to Managed cat old

If you go to the folder above the one you've selected, does it have a Managed Catalog inside as well? Have you renamed it?

No...I didn't realize that there was another folder with the managed catalog file inside...I'll rename it now and see what happens

You are amazing...yup...looks like it's working now.  Thank you so much.  What a great resource you are.  I'll reply to this thread if I have any other issues with this but for now, it looks like it's populating correctly. Many thanks

I don't even see my MCAT file :(

I'm struggling with this issue and have followed Rick's steps. I restarted, uninstalled, reinstalled, changed the mcat file to "Managed Catalog.old" and tried again. Repeated the process and Lightroom still wouldn't import folders, but created a new mcat. I changed that one to "Managed Catalog2.old" and continued ... Now I have Managed Catalog.old, Managed Catalog2.old, Managed Catalog3.old and the now newest mcat that Lightroom created. I'm not sure what else to try. I'm using a cloud based Lightroom and it gave me a major update a couple of weeks ago. It's been working well, but hadn't synced to my previous work. I don't know if not having this synced is part of my issue, or even how to sync it back up if that's necessary. Please help if you can! Thank you!

Hello I have the same issue. Did exactly all the steps mentioned above, changed the Managed Catalog.mcat to old, reinstalled, reset the preferences, restarted, and etc. The error message is still there and I can't import any images fro my camera anymore. Here are some screenshots:

Greetings Catherine,

Looking at your screenshot I would like you to try this. 
Close Lightroom.
Rename any file that is Managed Catalog.anything to backup.anything  with the goal being to have no files named Managed Catalog regardless of their extension. 
Launch Lightroom
Wait one hour before attempting to work

Do you see the message now?