App locks up on refresh after login -Vuejs

I set up Auth0 for Vue, following the instructions for a vue application in the docs. Things are working locally and on deployment, except for an issue that several others have had before: if I refresh the page, the app locks up, and the only way to get it going again is to manually delete the cookies.

The same issue is discussed here:

I’ve removed the trailing slashes from Allowed Web Origins and still no dice. All I get in the console is:
[HMR] Waiting for update signal from WDS…
Followed by:
Failed to load resource: the server responded with a status of 400 ()
And then eventually:
[Vue warn]: Error in created hook (Promise/async): “TypeError: Cannot read property ‘isAuthenticated’ of null”

Any advice? This is the last problem I’ve got; once this gets resolved I can deploy.

Edit: forgot to mention, but this is probably in the vein of what I’m looking for: Auth0 client is null in Vue SPA component on page refresh
Can anyone patiently dumb this down for a poor junior dev, down on his luck?

First question, which request is returning a 400 status? Is it a request to Auth0 or a request to your own back end?

Second, have you verified if the isAuthenticated property or Auth0 client object becomes valid after some time has passed? For instance, if you attempt to access the Auth0 client after navigating to some other page (not refreshing again), is it valid?