Follow-up:After a while the initial "power up for a quarter a second" phenomenon arose again
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After unmounting the rig, measured PSU 12 V: was present during that quarter second, but then was again shut down spaneously. To retry this test you first have to unplug the IEC mainsconnector wait and reconnect.
Unplugged the MoBo ATX connector and pushed the main switch. Power on remained. Then switched off and replugged the ATX. The scope restarted normally!
Therefore I decided to recap the MoBo.
And also the acquisition board ones (especially those tantalums beginning to fail in shorted mode yelding higher power consumption and supposedly overcurrent protection). While doing this I'll take care to select caps with at least a three fold voltage margin.