At this point, sorry not sure what your issue exactly is
Am I guessing correctly that your FB live stream works fine. it is just that in /Live/Producer, you aren't seeing the preview window?
if so.... nothing to do with OBS... so we'll see if I or someone else can help, but you should be asking Facebook (yea, I know.. but there are forums focused on their platform that are likely to get better response than on an OBS forum]
1. FB web page errors... happens... happens to me sometimes.. if preview, not working, I usually move on and ignore it... or you can change stream health, or scroll up on stream preview to get larger window view..
2. I've noticed that certain FB features related to videos (at one time, last year) only appeared when using Chrome, and not MS Edge (Chromium) version... so I made sure to use Chrome when connecting to FB, and haven't gone back to see if FB fixed their code
I bring this up, as you don't mention which browser you are using