Google Assistant keeps saying "something went wrong while syncing with Home Assistant", please help!

I’ve been using the Google Assistant component (manual) for months with no problem. Since upgrading to 2021.7, the service has stopped working. It keeps saying it can’t reach my Home Assistant, when I can clearly see the request in my NGINX logs. I’ve tried downgrading back to 2021.6.6 with no luck. I’ve also tried setting up a new project on Google and unlinking and relinking the accounts. google_assistant.request_sync runs successfully (no errors in logs). I can see the devices in my Google Home, but they are all unavailable. There are no errors or warnings when calling for HA via Google Assistant, but I found the following error logs using Google’s Logs Explorer:

{
insertId: "j5fumkg35ahwae"
logName: "projects/<project_id>/logs/actions.googleapis.com%2Factions"
receiveTimestamp: "2021-07-27T20:29:54.359315165Z"
resource: {
labels: {
action_id: "SMART_HOME_QUERY"
project_id: "<project_id>"
version_id: ""
}
type: "assistant_action"
}
severity: "ERROR"
textPayload: "requestId 3388242384554250071 failed with code: BACKEND_FAILURE"
timestamp: "2021-07-27T20:29:54.348367572Z"
}

I really haven’t changed anything else other than updating the system, which is now reverted. Does anyone have any ideas on what to do?