I have configured jUpgrade Pro as follows to do the migration:
I then tried an initial transfer of ther users. Bummer ... jUpgradePro seemed to freeze.
For my second attempt I set jUpgradePro to use RESTful. After configurating it, I installed the free restful system plugin in the 1.5 website. Then I tried starting the migration of the users. That did the trick. Users and groups are now migrating.