Google Assistant integration - 404 when trying to add to Google Home app

Tags: #<Tag:0x00007f3273d4f970>

My Google Assistant has been working fine (in manual mode), but recently I noticed that the account had become unlinked in my Google Home app. I followed the instructions at the link below to set up my app again on the Google Actions Console:

…but when I got to step 6 (adding to Google Home) I got a 404 error. The browser had opened with the correct URL (and I could access the same URL from a Chrome browser on the same phone), so I have no idea why it is throwing the 404 error when trying to link the account in Google Home.
I’m running HA 0.65.0 (Docker) on a Linux VM.
Any ideas for how I can diagnose/fix this?

I provided the following info on the Account Linking page on the Actions Console:
Authorization URL: https://[mydomain]:8123/auth/authorize
Token URL: https://[mydomain]:8123/auth/token