I have the same problem and tried to install Tbird v52.0 from v 52.1 but no luck. Am going to go one version lower to see if it fixes issue.

The fix for me was to install Tbird v45.8 (down from 52.1.0 and 52.0 which didn't work) and related Lightening file v 4.7.8. Need to get older versions from: https://archive.mozilla.org/pub/thunderbird/releases/45.8.0/win32/en-US/ and https://addons.mozilla.org/en-US/thunderbird/addon/lightning/versions/4.7.8

You might want to backup your current install before rolling back in case a boo boo happens! I use Mozbackup.

Thanks Bob22!

I guess that workaround would do it for me, too. However, I'm still hoping that this bug will be fixed soon, so I'm waiting before I do a downgrade. In the meantime I'm using the web version of the Exchange calendar that's relevant for me for inviting other people. Not a perfect solution either, but currently I can live with this situation ...

The button to "notify attenddes" is greyed out for me too. When I right click on the calendar entry, there is to "compose message" but no details (date/ time/ location are embedded in the message. What point is there to keeping Thunderbird updated if we have to revert back to previous versions to get something working again?

My fix of this problem follows. As of July 24, 2017, my Tbird v.52.2.1 is working mostly as desired. I also have Lightning 5.4 and Provider for Google Calendar 3.3 installed. 1. I did a full backup of my Tbird (profile and messages). 2. Uninstalled Lightning and Provider for Google Calendar add ins. 3. Downloaded and installed latest Tbird update. 4. Restarted PC (just because). 5. Downloaded and installed Lightning 5.4 and Provider for Google Calendar 3.3. 6. All works again except when I make a change or send an invitation to someone it doesn't seem to do it but when I hit "Save" it does.

I hope this helps you! I almost went to Outlook as I was fairly fed up.

Thanks bob22, I will try your suggestion. But I wonder what goes wrong with the update process to cause the problem. Advice, don't even try Win 10 Mail (replacement for Outlook Express). It works like it was a homework assignment for a school project that was rushed at the end of the deadline. There is no means to import or export. It has its own Contacts list that does not refer to the pre-existing Windows Contacts folder. Yes, it syncs with online mail provider contacts, when on line, and no way of setting a contact fro the on line mail box as local. I do not think that Tbird (or any other client) would be a import from it.

Thanks for the note on Win 10 Mail.