HP Slimline 260: CPU fan error (90B)

I document this as I run such metal as headless server not having the CPU fan connected to the main board. It is low load and up to now, I can not complain about overheating. But from time to time, I need a reboot and I tend to forget the magic to make it start without connecting a monitor. And as it happens, I lately forgot to connect the keyboard to this piece of metal. So next time, think of it! Be it as it may, to make it start one has to press ENTER within 15 seconds, however I completely ignore from what exactly.