Error when accepting a calendar invite on Thunderbird: `Processing message failed. Status: 80004005.`

Steps to reproduce

1. Receive a calendar invitation
2. Click on accept
3. Error happens: "Processing message failed. Status: 80004005."

Expected behaviour

The event should be added to the calendar and the answer sent to the organizer

Actual behaviour

I’m experiencing this on Mozilla Thunderbird 68.2.2. This problem happens when I receive an invitation. Sometimes, I see the HTML message (with the Nextcloud logo…). In this case, the error message 80004005 appears.

At some point, I don’t know why, the email looks different, with only one single table, with a blue row on top with the title of the event: when the email turns into this format, I can accept an invitation without any problem.

It happens if the person sending the invite is using Thunderbird, and is using a calendar on the same Nextcloud server. And it can also happen if the sender is not in my organisation. (using Outlook or Thunderbird, I don't know)

It might be related to the server managed scheduling mode: (on thunderbird, I have calendar.caldav.sched.enabled = false

In my config.php, I see:
'app_install_overwrite' =>
array (
0 => 'calendar',
),

Should I remove that?

I see related issues here:

• https://help.nextcloud.com/t/accepting-invitation-with-thunderbird-lightning-and-nextcloud-12-0-5/26791
• https://help.nextcloud.com/t/accepting-meeting-invite-fails-due-to-bad-outlook-formatting/55170 and the corresponding Github issue: https://github.com/nextcloud/server/issues/16679
• https://github.com/nextcloud/server/issues/8423
• https://bugzilla.mozilla.org/show_bug.cgi?id=1406402

Server configuration

• PHP 7.3.11-1+020191026.48+debian101.gbpf71ca0 (cli) (built: Oct 26 2019 14:20:37) ( NTS )
• mariadb Ver 15.1 Distrib 10.3.17-MariaDB, for debian-linux-gnu (x86_64) using readline 5.2
• Nextcloud 16.0.6

Start with the reproduction steps for accepting an invitation in Thunderbird 68.2.2 and compare the two message formats described in the issue. Check the server configuration in config.php, including calendar.sched.enabled and app_install_overwrite; done means the event is added to the calendar and the response is sent without status 80004005.