The mystery deepens. I brought it back to the repair shop some days ago, and got an email from the professional involved, today. He writes that they have the Io up and running once more, but he wants to keep it for further testing. He writes (my translation from Norwegian);
"I changed the fuses that had gone from 'fast' to 'slow' 0.5A. Did not find the BUSS [US] fuses that I thought I had, but found the documentation for those fuses and as far as I can see now it should be 'slow' 0.375A. For now, everything looks fine. Has been left on, for testing, yesterday and so far today. Want to test some more for checking power voltages esp around the regulator."
"I changed the fuses that had gone from 'fast' to 'slow' 0.5A. Did not find the BUSS [US] fuses that I thought I had, but found the documentation for those fuses and as far as I can see now it should be 'slow' 0.375A. For now, everything looks fine. Has been left on, for testing, yesterday and so far today. Want to test some more for checking power voltages esp around the regulator."