For the past week or so, there's a small orange error symbol in the top right corner when I open Chrome. It says to "Update sync passphrase". I don't remember ever having to enter a passphrase for this. It says I can reset it via the Google Dashboard but won't that delete passwords, autofill among other things?
If you've changed your Google Account password, then the sync passphrase will be your Google Account password prior to the one you changed to.
So if you changed your Google Account from hunter2 to hunter3, then the sync passphrase will be hunter2