Win or Mac? Other computers on same router network CAN open, just your localhost attempts fail?
Create new user, log in as new user and test?
Meaning on your computer, database open, IWP on, browser open.
Make a new database from template, turn on IWP and test to eliminate possible causes?
Some Links about IWP
FM runs on MAC. I can use IWP from a windows PC on the same network.
http://localhost:591 does not work at all.
http://localhost brings up the database screen. Clicking on the database brings up the lgoin screen.
I tried another database without a difference in behaviour
Reinstalled FM with out difference
Deleted the preference files with out the difference