ATX Power Supply delivers zero current? [Deleted]

I have an ATX power supply (taken from an HP Pavilion desktop) that has worked well in many projects, with or without the recommended dummy load across a 5V lead. I have used it to drive circuits requiring 5V, 3.3V, and 12V. But recently I tried to deliver +6V=Vcc/-6V=GND to an opamp (LM358) with the help of a voltage divider and the 12V PSU output, but this doesn't work: the current seems to drop to zero. Everything works fine when I use a battery, or a 12V wall wart (AC adapter) to deliver...

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