Unfortunately a bug made it into Unreal 5.2 in the FBIK solver. If you only have 1 effector, it will flip out and produce NAN’s. The fix can be made in PointsToRotation.h (see line 20-25 in the image).
This is a regression after we improved the ability of the solver to rotate the root of the skeleton based on effector motion when using “Pre Pull” method. With the ability to adjust the amount of rotation. Before it only affected the position of the root.
Apologies for any inconvenience. This will be resolved in a subsequent update, but I wanted to ensure the community was aware.