Incompatible browser error
Please check you are not using an incognito window as a first step.
Please check you are running the latest version of the browser you are using. We always test the WMT vault in the latest version of popular browsers.
Sometimes extensions that block certain browser behaviours can block "real" services. So consider disabling some of your browser plugins, especially the ad blocker, any security plugins, or VPNs, that would be a good start.
Check your browser is allowing local storage. The vault uses some browser storage to allow users to unlock their vault using a passphrase. There is a guide here to check Chrome's local storage is enabled here: https://techglimpse.com/enable-localstorage-support-google-chrome-browser/