I had to update the Babyface, elicenser and ilok drivers + re-authorise a handful of plugins. Also, some file associations were lost. But the one big issue was that 1903 wiped my “My Pictures” folder. This is a known, albeit rare, issue, so if you have assigned a new location for any of these “special” folders (My documents etc), make a backup before updating. I was super-lucky, since I swapped out my doc-HD the week before. I could restore 99.9% of the content from the old one.
Major issue #2 was that the update killed my nvidia drivers, reverting to the default drivers for gfx. At first I didn’t notice it, but quickly understood what had happened when every plugin window took 10 secs to open. Reinstalling the geforce drivers fixed it.
Otherwise all now runs smoothly, on two systems.