This is 2400 memory. Despite what CMTL says, any speed above 2133 could result in improper operation. One NUC could work while another (seemingly identical) does not. In the simplest of terms, the NUCs are design to support only 2133 memory; no formal validation with faster memory is taking place.
When you boot with the BIOS configuration jumper removed or moved from the 1-2 position to the 2-3 position, the system will be powered up in a compatibility mode where the memory is configured for base operation (i.e. 2133). This is why you were able to be successful doing some things.
So, what to do? You can try manually setting the memory configuration to operate at 2133, but many folks have reported little success in doing so. A better thing to do is to try with true 2133 memory. If it works, you will know that you need to replace the 2400 SODIMMs. If it doesn't work, you can then look into RMA'ing the unit.
Hope this helps,