Again the (incremental) update via Updater app failed (like the upgrade from 5.1 to 6.0 and the first update from 17.094 to 17.05). This time, however, I’m not sure why it the Updater app had failed: this time I hadn’t deactivated any standard / Google apps, so basically everything should have been fine.
I had to flash the binary images from my computer (method 4) like described on the Fairphone Support page, which worked very well.
Maybe deletion of Updater app cache and data would also have done, but since I had the phone connected to my computer anyway (for the non-installed-TWRP backup, thanks again to @retsifp for this description), I reasoned this was the fastest and cleanest way to update.
Hopefully the improved Updater version coming with this update will improve the situation.