Meanwell RT125D supply behaviour
Looks like it was the over voltage protection on the 5Volt output that caused your problems.  

Unloaded, the 5V line went over 5.75 Volts, causing the power supply to go into "hiccup" mode where it shuts off the output, waits a while, then tries it again, over and over until the over voltage condition goes away.

The datasheet doesn't have an explicit entry for the minimum load for 5V, but it is rated for 2A to 15A (maximum), nominal 8A.  So, it probaly needs a relatively large minimum load to behave correctly.  

2A at 5V is a resistance of 2.5Ohms, so pick something that needs a lot of power.
If you try with a resistor, it would need to be rated for 10Watts.