I have bought a new high end laptop for our sports live stream. An i7 with an Nvidia GeForce RTX 3050 graphics card. I have configured the new laptop's power management options for high performance.

From the old computer, I have copied over the same preset that I have been running with for several years. Everything works perfectly on the new computer with all settings, I've triple checked. But when I start the live streaming, the "Stream" button lights up red for one second, but then continues with flashing yellow (by flashing meaning a static yellow color, but red is flashing several times a second).
When I preview the broadcast, the live picture also stutters several times per second.

I'm running with only one camera, it's set to 1920x1080@50p all the way from the camera to the output and the streaming setting. If I switch back to my old slow computer, with the exact same settings and on the exact same internet cable, streaming to the exakt same platform, then no problem, the "Stream" button is constantly red, with smooth pictures on broadcast preview - as it always has been. The upload speed is set to 6 Mbps and has been working for years.
I also usually use Record and Instant Replay at the same time, but even without them running, the problem remains. I've checked the CPU and GPU percentage on task manager during the broadcast, both are very low.

Is the network card in the new computer defective? Surfing with it works without problems. I have run broadband tests, no problem with it, it shows almost 1000/1000 which is what we have here.
I have spoken to supporters for the platform I broadcast on, they have no answer to the problem.
Is there something (obvious) that I missed?

Thanks for the reply and advice!

I'll have a look at the logfile if I can spot anything obvious in it.

Where can you make settings so that it uses the graphics card that is in the computer and not a card that does not exist? I can't find anything in the vmix documentation about that.

I also notice that even though the CPU and GPU in the activity manager show a fairly low percentage, a 15 sec replay is shown for 25 sec with 100% speed. So something is really wrong.

I will try to make a new, clean and simple preset with only one camera input, nothing else. Then we'll see if it works better.

Thank you, Mavik!!!
After Congoblue's answer, it got me on the right track, this was exactly what I needed to move forward!
That was the cause of my original problem. It turned out that when I read in Settings--Performance--Graphic Adapter, it showed "In Use: Intel(R) UHD Graphics" which was the wrong card. But I was lost here.

Then, thanks to the instructions on your link, the right card appeared: "NVIDIA GeForce RTX 3050".

Still, after a restart, it was pretty jerky. I updated the graphics card driver, restarted the laptop, then it worked flawlessly. What a difference!

I also tried running my copy of the preset from the old computer. I started Recording, used Instant Replay and also a livestream test, all at the same time. CPU peaked at 35-40%. The "Stream" button was steady red, no yellow flashing, just steady red. Finally!

The only strange problem now was that the Camera Input showed a stuttering image for a few seconds which then froze.
The weird thing was that it only did it when I panned the camera, when there was movement in the camera image. When it was still, all worked.
I have the camera connected to a Magwell HDMI to USB3 (Gen 2).
I resetted the input on vmix, but same thing again. I restarted the computer. Same problem.
I tried in another usb port. Same problem. I tried in the third usb port (the laptop has three usb 3 ports). Then it worked without problems. I tried the first port again. No problem. I tried all three usb ports. No problem.

What happened in the beginning? Will it happen again? It feels unstable to dare to have in a live broadcast. It feels like I first have to test it for a longer period of time to be absolutely on the safe side.

The new computer is an ASUS TUF GAMING F15 FX506HE-HN001W
Do I need to update the drivers for the usb ports? What if they already are up to date?

EDIT: I've using the computer and my preset now for many occasions and hours. No problem, very low cpu and gpu percent, it works flawlessly.
Conclusion: Changing card from "Intel(R) UHD Graphics" to the correct "Nvidia GeForce RTX 3050" was the solution.