On both this, and an old MacBook Pro, I have had the same issue: whenever I connect to any public WiFi, the captive portal window appears blank, with an error message stating that the page could not be loaded.
There are a few solutions offered to a slightly different problem – to bypass captive portals altogether – here, but none of the popular solutions offered work as of 10.14.3.
The issue in my case is slightly different as I just never see anything in the portal. Instead, every time I want to log in to public WiFi, I have to open a browser, close the 'blank' captive portal (which will keep popping up with error messages), and then type 192.168.0.1 manually into the browser. The page will only load in the browser if I have managed to keep the 'blank' portals from being open at the same time.
How can I log into a captive portal without having to type 192.168.0.1 into a browser every time? The fact that I have had the same issue on two different MacBooks would suggest that the problem is quite widespread.