Somehow I missed the introduction of the new Teams and Skype for Business Admin Center (See What´s new for IT Admins in Microsoft Teams). However, I did notice that my Chat feature disappeared. Originally, I had a different Chat setting for faculty and students. Faculty could chat one-on-one, students could not.
With the introduction of the new admin center, one has to apply policies to the different users. Somehow, many of my users did not have any policies applied, which would then exclude them from setting meetings and chatting. This may have coincided with the fact that we started using AD for our users (which meant "new accounts").
Be that as it may, I'm just bringing this to your attention in case you too experienced a change in chat policy with the move to the new admin center.
The new admin center looks OK, I'm just missing a way to allocate a policy to a group of users. It's probably there but it is not obvious to me how to do it. In the Office 365 Admin center one is able to filter the users in a custom view and then apply changes to all those selected. I can filter in the new admin center, but I cannot select all of those users to apply a policy.