It looks like the problem was in the system>>settings>>configuration. 8.3 used to accept that configuration, but 8.4 does not like it.

Not sure how to repeat that. If I remember it correctly, the configuration had "number of concurrent users">"number of named users" and as a result all named users were "eaten" by concurrent users and needed to "eat" even more named users, but there were not enough named users to "eat", if I understand the licencing process correctly.