Changes to a channel cannot be viewed or discarded anymore

Changes to a channel cannot be viewed in the 'Manage changes' window' anymore. The list of changes in 'Manage changes' is always empty, whether changes have been made or not.

Changes can also not be discarded anymore. Clicking the button 'discard changes' does not discard the changes made, and the message 'You have unpublished changes in this channel' is still shown. The message also does not go away after a hard refresh or logging out and in again.

Steps to reproduce:
1. login as admin at cms.demo.test.onehippo.com
2. open the English Website
3. make a small change
4. click 'Manage changes'

Expected: you are listed as one of the users that has made changes in this channel
Actual: the list of users with changes is empty

5. click 'Discard changes'

Expected: the change you made is discarded
Actual: the change you made is still there

The '/discard' REST call is executed and returns OK, but apparently does not do anything anymore.