Which browser are you using? F12 open the developer tools, and look at the browser console log.
And what version homeassistant are you running?
After an attempt to upgrade my hass.io system by initialising a new SD card I attempted to return to my original installation. However I am now encountering the same "Unable to connect to Home Assistant. " screen as @myxu13 advised at the head of this thread.
I noted that it would be worthwhile reviewing and uploading the log file but given I’m unable to access the server I don’t know how I would access it.
I tried @flamingm0e’s “incognito mode on Google Chrome” suggestion but that didn’t work.
Any other suggestions would be gratefully received.
I have the same issue