User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:93.0) Gecko/20100101 Firefox/93.0

Steps to reproduce:

Start Thunderbird.
Go to "Error Console".
Observe error.

Actual results:

Error observed.

Expected results:

No error.

Does it happen every time? And does it happen without the Google Provider add-on enabled?

I see it again, this is after updating to 91.1.2 (64-bit). There are other "Assert failed" message further down the "Error Console" as well.

After turning off all addons/extensions, I do not see the "Assert failed" messages. However, I do still see:

10:06:12.466
Unexpected event profile-after-change URLQueryStrippingListService.jsm:224
observe resource://gre/modules/URLQueryStrippingListService.jsm:224

10:06:14.888
Unknown Collection "thunderbird/query-stripping" RemoteSettingsClient.jsm:160
UnknownCollectionError resource://services-settings/RemoteSettingsClient.jsm:160
sync resource://services-settings/RemoteSettingsClient.jsm:501
InterpretGeneratorResume self-hosted:1482
AsyncFunctionNext self-hosted:692

Addons/Extensions (all now disabled):
Compact Headers
Display Mail User Agent T
LookOut (fix version)
Provider for Google Calendar
Remove Duplicate Messages

I also get this after switching to ical.js on TB 91.

Does this need to block 978570 ?

I don't know of any reason why switching to ICAL.js would cause this, and it doesn't happen to me. I could understand how an add-on would cause it in the original report, but in any case it's not telling us that anything broke, just that something happened before we expected it (which shouldn't happen but isn't a problem otherwise).