Custom domain 504 Gateway Timeout for some users when requesting a token

Since we started using a custom domain this weekend some users get a 504 response when requesting a new token. We experience no errors for other users. One specific user gets the error, both in Chrome and Edge, so it doesn’t seem to be browser related.

The SSL certificate for that domain got refreshed also this weekend.
The custom domain test in auth0 tenant settings is successful.

Where do we start to find the cause of this?