net6501: doesn't start if not first unplugged for 8 minutes

Dear Soekris wizards,

I have a net6501 soekris device and it has a strange issue: when the
device is working correctly and I reboot, then it doesn't start
anymore: the red error led stays on and there's no output on the
serial port. A reset by pressing the reset button or a reset by
disconnecting and connecting again the power source does not make a
difference: the device simply does not start anymore. The only thing
that still works is the uManager Monitor that I can access with '+++':
I can even upload the latest rom and issue commands like 'power cycle'
but in the end to no avail: the regular bios/pxeboot/os does not
start.

If I keep the device unplugged from a power source for +/- 8 minutes,
then it does start again and everything works like it should.

The device has been running uninterrupted for years without a reboot
in the 19" case with built-in power supply that is sold by Soekris.
I've also tried now with some other 12V power supply but it doesn't
seem to make a difference.

Do you maybe know what could be the problem? Any suggestions on what I
still could try or how I might be able to solve the problem?

Kind regards,
Dries