Streamlabs has recently released an update for their version of OBS (Streamlabs Desktop) and it seems to have broken game captures for many users. They have announced that they are aware of the issue and are working on a fix since November 17th, 2023.
What can be done while waiting for them to fix it?
One user has reported:
"Open streamlabs and it won't work, and while it's open you open OBS as well, it'll work there. Once it does you just close it and streamlabs should magically work xD"
Otherwise:
1) You can use regular OBS instead
OR
2) you can use window capture instead of game capture. (Window capture requires you to select a background color for stream avatars, and then use that color in the window capture filter option (Color Key). You can adjust it so the background is transparent)