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There was no screenshot. Please provide it, since that will be the best way for me/us to see your situation. How do I create a screenshot of my problem?

Trying again.

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I recommend first running the Disk Utility to check for file and hard drive problems: How to run "Disk Utility" in Mac OS https://support.apple.com/en-ca/HT201639 http://osxdaily.com/2012/05/24/check-hard-drive-health-mac-disk-utility/

According to Disk Report, I have no problems.

have you turn the calendar back on by ticking it? I notice it is not active in the image you posted.

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Next thing is to backup your calendars, remove the Lightning add-on, reboot the computer, add Lightning back, import the backed up calendars.

1. Find & open your Thunderbird Profile folder: Thunderbird menu: Help: Troubleshooting Information Under "Application Basics", beside "Profile Folder", click the "Show Profile" button.

2. Navigate to the "calendar-data" folder and copy it to another location for safekeeping (backup).

3. Thunderbird menu: Tools: Add-ons: Extensions Find the Lightning add-on and click the "Remove" button for it.

4. Quit Thunderbird.

5. Reboot the computer.

6. Start Thunderbird, then add the Lightning add-on again. Thunderbird menu: Tools: Add-ons Type "Lightning" in the search, then click "Install" button for it.

7. If necessary, copy back the files you backed up from the "calendar-data" folder.

8. Refer to my step 17 here for how to tell Thunderbird to open the calendars.

Matt, I have 13 calendars that are supposed to be displaying.

Bruce, I followed the instructions. Now, the calendar tab flashes back and forth rapidly (4-10 times per second) between the two attached views.

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Next step: Create a new profile in Thunderbird for testing

If the calendar still doesn't work properly in the test Profile, then the next step would be backing up your profile, uninstalling Thunderbird, rebooting, installing Thunderbird with Lightning.