Just reporting back, we found out what the problem was.
When upgrading from Moodle 3.1 (and older Adaptable version) to Moodle 3.4 (and latest adaptable version) it seems to be essential to empty all local cache/cookies/etc. on a machine.
We have tested this with Apple/Safari/Chrome and Windows Chrome/Edge/IE. They all had the same problem. Somehow, part of the adaptable scripting stays on the local machine and this makes that the buttons 'hide blocks' and 'fullscreen' do not work.
Locally deleting history and cookie information solves this problem.