User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:69.0) Gecko/20100101 Firefox/69.0

Steps to reproduce:

1. Create a calendar event with at least one attendee who is not me.
2. Receive an Event Invitation Reply from attendee (e.g., "Event Invitation Reply (Accepted): coworking").
3. Notice the "This message contains a reply to an invitation." bar above the message and click "Update" button.

Actual results:

A dialog box pops up with no title or content, but with "Cancel" and "OK" buttons and an X button in the upper right to close the box. Clicking Cancel or OK does not result in any change or in closing the box. Clicking X does close the box.

Console:

Stylesheets without CSSOM changes reparsed to check for errors. Refresh the page to also see errors from stylesheets changed from CSSOM and from style attributes.
Unknown property ‘user-select’. Declaration dropped. reps.css:325:15
Stylesheets without CSSOM changes reparsed to check for errors. Refresh the page to also see errors from stylesheets changed from CSSOM and from style attributes.
PUT https://caldav.[redacted]/[redacted].ics
[HTTP/1.1 412 Precondition Failed 259ms]

Request URL:https://caldav.[redacted]/[redacted].ics
Request method:PUT
Remote address:[redacted]:443
Status code:
412
Version:HTTP/1.1

Response headers (245 B)
Raw headers
Accept-Patch
text/calendar; charset=utf-8; …CH; optinfo="PATCH-VERSION:1"
Cache-Control
no-cache
Connection
keep-alive
Content-Length
0
Date
Thu, 06 Jun 2019 19:48:15 GMT
Server
nginx
Request headers (644 B)
Raw headers
Accept
text/xml
Accept-Charset
utf-8,*;q=0.1
Accept-Encoding
gzip, deflate, br
Accept-Language
en-US,en;q=0.5
Authorization
[redacted]
Cache-Control
no-cache
Connection
keep-alive
Content-Length
1206
Content-Type
text/calendar; charset=utf-8
Host
caldav.[redacted]
If-Match
"[redacted]"
Pragma
no-cache
User-Agent
Mozilla/5.0 (X11; Linux x86_64…hunderbird/67.0 Lightning/6.9
SyntaxError: redeclaration of var cal calendar-conflicts-dialog.xul:17:1
<anonymous> chrome://calendar/content/calendar-conflicts-dialog.xul:17
promptOverwrite resource://calendar/modules/utils/calProviderUtils.jsm:338
promptOverwrite jar:file:///home/sumanah/.thunderbird/[redacted].default-beta/extensions/{[redacted]}.xpi!/components/calDavCalendar.js:579
onStreamComplete jar:file:///home/sumanah/.thunderbird/[redacted].default-beta/extensions/{[redacted]}.xpi!/components/calDavCalendar.js:812
onStreamComplete self-hosted:993
oauthCheck jar:file:///home/sumanah/.thunderbird/[redacted].default-beta/extensions/{[redacted]}.xpi!/components/calDavCalendar.js:355
oauthCheck self-hosted:1037
ReferenceError: onLoad is not defined calendar-conflicts-dialog.xul:1:1
onload chrome://calendar/content/calendar-conflicts-dialog.xul:1
promptOverwrite resource://calendar/modules/utils/calProviderUtils.jsm:338
promptOverwrite jar:file:///home/sumanah/.thunderbird/[redacted].default-beta/extensions/{[redacted]}.xpi!/components/calDavCalendar.js:579
onStreamComplete jar:file:///home/sumanah/.thunderbird/[redacted].default-beta/extensions/{[redacted]}.xpi!/components/calDavCalendar.js:812
onStreamComplete self-hosted:993
oauthCheck jar:file:///home/sumanah/.thunderbird/[redacted].default-beta/extensions/{[redacted]}.xpi!/components/calDavCalendar.js:355
oauthCheck self-hosted:1037
TypeError: this.gViewSourceUtils is undefined webconsole.js:168:5
ReferenceError: onAccept is not defined dialog.xml line 434 > Function:3:1
anonymous chrome://global/content/bindings/dialog.xml line 434 > Function:3
_fireButtonEvent chrome://global/content/bindings/dialog.xml:435
_doButtonCommand chrome://global/content/bindings/dialog.xml:403
_handleButtonCommand chrome://global/content/bindings/dialog.xml:391
promptOverwrite resource://calendar/modules/utils/calProviderUtils.jsm:338
promptOverwrite jar:file:///home/sumanah/.thunderbird/[redacted].default-beta/extensions/{[redacted]}.xpi!/components/calDavCalendar.js:579
onStreamComplete jar:file:///home/sumanah/.thunderbird/[redacted].default-beta/extensions/{[redacted]}.xpi!/components/calDavCalendar.js:812
onStreamComplete self-hosted:993
oauthCheck jar:file:///home/sumanah/.thunderbird/[redacted].default-beta/extensions/{[redacted]}.xpi!/components/calDavCalendar.js:355
oauthCheck self-hosted:1037

ReferenceError: onCancel is not defined dialog.xml line 434 > Function:3:1
anonymous chrome://global/content/bindings/dialog.xml line 434 > Function:3
_fireButtonEvent chrome://global/content/bindings/dialog.xml:435
_doButtonCommand chrome://global/content/bindings/dialog.xml:403
_handleButtonCommand chrome://global/content/bindings/dialog.xml:391
promptOverwrite resource://calendar/modules/utils/calProviderUtils.jsm:338
promptOverwrite jar:file:///home/sumanah/.thunderbird/[redacted].default-beta/extensions/{[redacted]}.xpi!/components/calDavCalendar.js:579
onStreamComplete jar:file:///home/sumanah/.thunderbird/[redacted].default-beta/extensions/{[redacted]}.xpi!/components/calDavCalendar.js:812
onStreamComplete self-hosted:993
oauthCheck jar:file:///home/sumanah/.thunderbird/[redacted].default-beta/extensions/{[redacted]}.xpi!/components/calDavCalendar.js:355
oauthCheck self-hosted:1037

Expected results:

I assume this is meant to be the "state changed on the server; discard my changes or push anyway?" box. So that copy should display, and clicking "cancel" should discard my update, and clicking "push" should push my update via CalDAV.

Thank you for the report. Based on given error messages it seems to be the same as bug 1552700.