I have seen this behavior in the pas with the Mail app, when it was in an early stage of development.
Maybe you can give us some more infromation:
- open the developer tools in your browser (F12)
- go to the console tab and see if there are any errors (a screenshot would be good)
- go to the network tab and see if there are requests which end in status 500 (a screenshot would be good)
You could also try to clear the localStorage in your browser, to do this again open the console tab and run localStorage.clear()
EDIT: I’ll try to install the mail app later today.