Did you try 127.0.0.1 instead of ‘localhost’ as well? Or your local IP address in the network? sometimes the DNS resolution is running weird.
If that doesn’t work, please post the contend of your dump1090 config file.
A little bit of an older thread, but maybe helpful as well:
Unable to access Web interface - FlightAware / ADS-B Flight Tracking - FlightAware Discussions