User Agent: Mozilla/5.0 (X11; Fedora; Linux x86_64; rv:75.0) Gecko/20100101 Firefox/75.0

Steps to reproduce:

I'm subscribed to my gmail calendar. For a lot of new recurring events, it's impossible to dismiss the event from Lightning.

I have been experiencing this issue for the last few days. Uninstalling and reinstalling Lightning 68.9.0 doesn't fix it.

dismiss the calendar event, or the reminder alert?

This turned out to be an issue while Provider for Google Calendar wasn't installed.
Specifically, the events were viewable and reminder notifications appeared, but were impossible to dismiss (I think there was some error in the JS console?)

There should at least be some error message made easily visible to the user, trying to nudge them to use the appropriate addon (not obvious)

Looks like this is actually back with a vengeance, with Provider extension still installed.

When trying to dismiss a reminder, I see this in error console:

Lightning: [calGoogleCalendar] Request PATCH https://www.googleapis.com/calendar/v3/calendars/[$USER]%40gmail.com/events/[...]?sendNotifications=true responded with HTTP 403
Lightning: [calGoogleCalendar] Login failed for [$USER] HTTP Status: 403 Reason: forbidden

Thunderbird & Lightning 68.10.0
Provider for Google Calendar 68.2.1

The same here for me since August 17, 2020 when I try to dismiss repeating events (Birthday reminders) from a network calendar. The calendar server is an Open-XChange system, accessed via CalDav.
My system: macOS 10.15.6, Thunderbird 78.1.1 and 78.2.0 (64 bit), with TBSync add-on. Everything is kept up-to-date.

Error console output:
TypeError: impl is null
network-response-listener.js:104:7
_forwardNotification resource://devtools/server/actors/network-monitor/network-response-listener.js:104
onStatus resource://devtools/server/actors/network-monitor/network-response-listener.js:395
TypeError: impl is null
network-response-listener.js:104:7
_forwardNotification resource://devtools/server/actors/network-monitor/network-response-listener.js:104
onProgress resource://devtools/server/actors/network-monitor/network-response-listener.js:391

Lightning: Parsing failed for parts of the item (while this is considered to be a minor issue, we continue processing the item):
[... some event details ...]
Parsing failed for parts of the item (while this is considered to be a minor issue, we continue processing the item):
BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//Open-Xchange//7.10.3-Rev20//EN
BEGIN:VEVENT
[...]
BEGIN:VALARM
TRIGGER;VALUE=DURATION:-PT15M
ACTION:DISPLAY
X-LIC-ERROR;X-LIC-ERRORTYPE=PROPERTY-PARSE-ERROR:Parse error in property n
ame: ACKNOWLEDGED
[...]
END:VALARM
END:VEVENT
END:VCALENDAR

Hint: could this be related to bug #1655245?

All these bugs seem to be describing the same problem. Not sure how to mark them as duplicates here so I'm listing them here.

https://bugzilla.mozilla.org/show_bug.cgi?id=1634617
https://bugzilla.mozilla.org/show_bug.cgi?id=1655245
https://bugzilla.mozilla.org/show_bug.cgi?id=1666156
https://bugzilla.mozilla.org/show_bug.cgi?id=1667087
https://bugzilla.mozilla.org/show_bug.cgi?id=1670526
https://bugzilla.mozilla.org/show_bug.cgi?id=1673219

I also can confirm this bug with my birthday calendar on Nextcloud. I have Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 with Lightning 68.10.0
It's a Linux Mint 20 Cinnamon.
I can not dismiss the event and if I just close the window it appears minutes later.
This is what I get on the error console:
Lightning: CalDAV: Unexpected status modifying item to Geburtstage: 404
BEGIN:VCALENDAR
PRODID:-//Mozilla.org/NONSGML Mozilla Calendar V1.1//EN
VERSION:2.0
BEGIN:VEVENT
LAST-MODIFIED:20201130T172155Z
DTSTAMP:20201130T172155Z
UID:5199fb4f-50f4-4838-be51-0b8eed487d45
SUMMARY:🎂 xxx (erased private information)
RRULE:FREQ=YEARLY
X-MOZ-LASTACK:20201130T172155Z
DTSTART;VALUE=DATE:19701110
DTEND;VALUE=DATE:19701111
TRANSP:TRANSPARENT
X-NEXTCLOUD-BC-FIELD-TYPE:BDAY
X-NEXTCLOUD-BC-UNKNOWN-YEAR:0
X-NEXTCLOUD-BC-YEAR:1955
X-MOZ-GENERATION:1
BEGIN:VALARM
ACTION:DISPLAY
TRIGGER;VALUE=DURATION:PT0S
DESCRIPTION:🎂 xxx (erased private information)
X-LIC-ERROR;X-LIC-ERRORTYPE=PARAMETER-VALUE-PARSE-ERROR:Got a VALUE parameter with an illegal type for property: VALUE=DURATION
END:VALARM
END:VEVENT
END:VCALENDAR

calDavCalendar.js:863

(In reply to Michael from comment #9)

I also can confirm this bug with my birthday calendar on Nextcloud. I have Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 with Lightning 68.10.0
It's a Linux Mint 20 Cinnamon.
I can not dismiss the event and if I just close the window it appears minutes later.
This is what I get on the error console, if I press the "dismiss" button:
Lightning: CalDAV: Unexpected status modifying item to Geburtstage: 404
BEGIN:VCALENDAR
PRODID:-//Mozilla.org/NONSGML Mozilla Calendar V1.1//EN
VERSION:2.0
BEGIN:VEVENT
LAST-MODIFIED:20201130T172155Z
DTSTAMP:20201130T172155Z
UID:5199fb4f-50f4-4838-be51-0b8eed487d45
SUMMARY:🎂 xxx (erased private information)
RRULE:FREQ=YEARLY
X-MOZ-LASTACK:20201130T172155Z
DTSTART;VALUE=DATE:19701110
DTEND;VALUE=DATE:19701111
TRANSP:TRANSPARENT
X-NEXTCLOUD-BC-FIELD-TYPE:BDAY
X-NEXTCLOUD-BC-UNKNOWN-YEAR:0
X-NEXTCLOUD-BC-YEAR:1955
X-MOZ-GENERATION:1
BEGIN:VALARM
ACTION:DISPLAY
TRIGGER;VALUE=DURATION:PT0S
DESCRIPTION:🎂 xxx (erased private information)
X-LIC-ERROR;X-LIC-ERRORTYPE=PARAMETER-VALUE-PARSE-ERROR:Got a VALUE parameter with an illegal type for property: VALUE=DURATION
END:VALARM
END:VEVENT
END:VCALENDAR

calDavCalendar.js:863

There are various dupes of these issue, consolidating them into one. I was going to file this one myself as well :-)