Hmm odd, seems it can’t find an IK solver in one of your IK Rig assets. I would check the asset that it’s sending to that function to see if something is wrong with it. Check its path, find it in your project, potentially even recreate it if needed.
This might also have been fixed in UE 5.2, you can check if upgrading to that fixes it.
If nothing helps then I would try to make a minimal repro project with only the crashing asset and send it to our bug submission form.