Thank you for trying.
I just flashed a card back to an old version and brought it back up to date as before. Streaming is working.
There appears to be some fringe condition I encountered to cause this. I'll try to catch what it is if it happens again. Until then, there is nothing more to do.

Well. This didn't take long. But it happened again. But this time I noticed it was right after configuring for guiding. It appears to happen to me when I configure it for external guiding with PHD2 or Linguider. Returning to internal guider setup corrects the issues with streaming.

When using external guiding, all binary transfer from cameras is disabled in Ekos UNTIL you try to capture an image. So try to do that, try to capture one image, then try streaming on. Does it that work?

EDIT: I updated KStars code to check if BLOB was disabled when video streaming is triggered. But test the above to ensure this was the problem.

Yes! There is a connection. Here's what I checked.
1. Tested that it was still not streaming - confirmed
2. Captured DSLR image
3. Test stream with GPhoto - Success!
4. Test stream with ZWO ASI 120 - Failure
5. Capture 1s exposure with ZWO
6. Test stream with ZWO - Success. But the FPS held a firm 1 FPS
7. Captures 0.5s exposure with ZWO
8. Test stream with ZWO - Success. Now the FPS held a firm 2 FPS

So the curious thing here is with the ZWO, the exposure time of the last image captured is actually applied to the framerate of the next stream.

By the way. Besides this bug. I have another fringe case I was wondering about. Does the stream system have a fixed cutoff time while waiting for incoming frames? For example if a frame takes more than 1 second to arrive, is it prompted to close the stream?
In my case I have the ability with MagicLantern to override the Canon 600D live view/video frame rate from 30FPS to as low as 0.2FPS. Greatly multiplying the light gathered per frame. But I can only stream to EKOS down to 1FPS. Any slower and EKOS gives up waiting for the frames.
Would it be possible to allow it to wait for up to 5 seconds for streamed frames?

hmmm I don't think there is a cutoff time. Have to check. For the ZWO ASI FPS issue, it is fixed in GIT.

Thank you Jasem. I'm very happy we could get to the bottom of this.
Having the streaming working properly will be very popular at the public star parties coming up.