Hi there @luke62 and welcome to the NVIDIA developer forums!
Look at the assertion and try to trace what object is actually active in either UpscalerInterface. That should point you in the direction of the upscaler you might still have active when trying to use DLSS.
It is not uncommon that when you use a project template or similar, that some default setting will activate the standard UE upscalers (like TSR )or AA tools, which will then cause this issue.
I recommend checking the UE documentation on these and see if anything in your project directly or implicitly enables one of them.
I managed to fix the crashing by changing Temporal Anti-Aliasing to (TAA) AND additionally changing the Project Setting ‘MSAA Sample Count’ to ‘No MSAA’.