Back home again. Now I have chosen the external volume with the fresh OS and PS install as the startup volume, and YES, now PS launches! - In the meantime, after 3 weeks of unexplained silence, the Adobe support also responded, suggesting a defect font might be the culprit. So I trashed all fonts from the main (=normal startup) volume and replaced them by the fonts of the fresh OS install. Now PS does not install onto the main volume... well, now Adobe seems to be back, so I hope they find a solution.
One strange thing: Even though I made no modifications of the fresh OS install, it contained 2 fonts that must have been my custom installed fonts of the main volume..?? (They were named with HB, and I recall that they were the result of a local support guy trying to recover the Arial font, of which the italic and bold versions had got lost.)
Thank you for your tips. Hening.