I don't know what is producing the gigabytes of error in syslog

I think the error is caused by VLC. Try using another Media Player.

"Paul_Pedant, I have googled but was not able to find anything useful".

I googled "org.gnome.Nautilus[1514]: [00007fa4fc465ce0] vdpau_chroma filter error: video mixer features failure: An invalid handle value was provided" and up popped four helpful posts.

Two of them concern syslog messages about "invalid handle value", and other two about "AssertionMessage: *** Error in `nautilus': free(): invalid pointer: 0x0993d258 ***". Those are probably the same root cause, because a pointer to free() is just a handle too. If Nautilus is corrupting its own memory, no telling what junk comes out.

https://bugs.launchpad.net/ubuntu/+source/nvidia-graphics-drivers-435/+bug/1846374

Having a video playing/paused when switched to another user generates gigabytes of error logs. syslog messages about "invalid handle value"

https://bugs.launchpad.net/ubuntu-gnome/+bug/1238993

nautilus assert failure: *** Error in `nautilus': free(): invalid pointer: 0x00007fc7ec3a8800 ***

https://bugs.launchpad.net/ubuntu-gnome/+bug/1266766

nautilus assert failure: *** Error in `nautilus': free(): invalid pointer: 0x0993d258 ***

https://ubuntu-mate.community/t/disabling-nvidia-driver-logging-errors/20258

Disabling nvidia driver logging errors

Maybe you should answer my detailed questions, or just uninstall and reinstall Nautilus, or check your versions against these bug reports, or file a bug on one of these sites. I don't have pop-os or Nautilus or Ubuntu or nvidia or a problem, so I can't investigate this further.

There is a bug filed for this at Having a video playing/paused when switched to another user generates gigabytes of error logs. In the bug description, it is mentioned that using a video player (e.g., VLC) is a way to pop the error. It is not clear if VLC is the only player that produces the error.

The bug was not solved. But as a way to stop the output, closing VLC apparently works. As per messages in the bug thread, and my own experience, avoiding having a playing video while switching users, and perhaps workspaces, should prevent the issue from appearing. The answer by nyxee is a related workaround.