Affected version(s): all
Since the installation of Thunderbird 78 or later the sync of the calendar isn't working anymore.
Because of the merge of the calendar plugin lightning into Thunderbird the user-agent changed from Lightning to Thunderbird. This matters only if you use the UserAgent method to recognize the traffic.
Have a look at the updated documentation:
You can add the RewriteCond for Thunderbird or use one of the other possible methods.
STATUS: documentation fixed