I will take a guess at what the problem might be but I could be completely wrong. Using Google Translate I was able to translate what the "environment" is supposed to be from Japanese to English. It reads...
1. Check the recommended environment for WOWOW On-Demand on your computer
You can check if WOWOW On-Demand can be viewed normally on your device by playing the video here .
Recommendedenvironment
・Windows 8.1 (Google Chrome, Firefox)
・Windows 10 (Microsoft Edge, Google Chrome, Firefox)
・Windows 11 (Microsoft Edge, Google Chrome, Firefox)
・Mac 10.12.6 / 10.13.6 or later (Safari, Google Chrome, Firefox)
The problem could be that StreamFab appears to them as if you were running Linux, which is not on the list. To be specific, the User Agent from StreamFab makes it look like you are using Firefox on Linux. If this is, in fact, the problem then this is just one more reason that StreamFab should contain a User Agent switcher. It has been requested many times. This is a common feature in many other video streaming downloaders.