The problem disappeared without my knowing what the cause was.

I managed to get into the BIOS setup procedure for the Phoenix MB at startup. There, I checked all parameters and noticed all components were properly recognized. The only things I changed and which appear to have no bearing at all on the 2/3 minute temporary freeze are the system clock (just 3 minutes slow, so the battery should be okay) and a parameter whereby I requested that the startup procedure stop when encountering an error. The existing option was to go on with the startup regardless of eventual errors.

Thank you for your suggestions.