There was a problem like this earlier this year and it seems to be related to some Nvidia drivers (I assume you have an nVidia GPU since the log states that NVENC is supported). For some people (me included) removing the libva-vdpau-driver package and replacing it with libva-nvidia-driver (from the AUR).
Others reported that a complete reinstall of nVidia drivers (switch to open-source driver → reboot system → switch to nvidia driver → reboot system) was the only way to solve it.
Here you can find the full thread: OBS-Studio not working after last update
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