Looks like this is reported, but for extra emphasis I'm getting an error after my BI upgrade this morning.
Yep, same error as everyone else. Blue Iris 4.5.6.0 and 4.5.6.1 has a broken web server. If you want to temporarily downgrade Blue Iris to the last working version while we all wait for a fix, I linked the 64 bit update installer here: 4.5.6 - May 17, 2017