In the Address book - edit-contact-private the month of April is missing and you have 2 March's.
When adding a birthday to a contact in the Thunderbird Address Book your calender has two months named March and no April.
When adding a birthday to a contact in the Thunderbird Address Book your calender has two months named March and no April.
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I'm using Windows and the current latest version of Thunderbird 52.9.1 I have no issue. as I use an addon extension 'MorefunctionsforAddressBook', I also checked in Thunderbird SAfe Mode and all is till working correctly.
What OS do you use? What version of Thunderbird do you use? Did you download Thunderbird from the official website ? eg: https://www.thunderbird.net/en-US/thunderbird/all/
I'm using 52.9.1 using Windows 10 and yes I downloaded TB from the official website. I'll look for the extension you mention and see if that fixes the problem.