have you restarted Thunderbird since the update to allow the updated Calendar to install?

Yes, I have restarted both windows and thunderbird. No change.

When do you see this error message?

It pops up all by itself. All it takes for the error message to appear is to have Thunderbird open.

What is even stranger is that the message box gives me the option to kill the process - which I always choose - but later on it pops up again. I guess TB restarts the process and it obviously crashes again.

Now I am starting to understand. Your getting a warning that a script is slow or has stopped. That is not a crash. Nor should you select stop. That message is like your phone beeping your later for a doctors appointment. and you cancel the reminder app instead of acknowledging the message.

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That's the pop-up dialog. TB was idle when this came up. As for cancelling it instead of telling it to try and restart the process, I used to tell it to continue.....and got exactly the same results!

By the way, thanks for your patience so far!

Rarely the script mentioned is actually relevant. but is can stop things like your mail being retrieved because you cancel a script.

What calendar do you have. I hear the Google birthdays calendar is a problem. What anti virus are you using?

Puzzingly, it has not acted out these past 2 days. Go figure.

Anyway, I do have Google birthdays calendar. If the problem returns, I'll disable that one first.

I use Avast, paid edition, always updated.

Thank you for trying to help, Matt! I will come back here if it starts again to inform on the Google birthdays calendar issue or to ask for further assistance.