I am having this same problem. I have tried to click on the save change checkbox and it still reverts to the same display name. I think this is a bug. I am using NextCloud 18.03 and I have tried to edit the users display name in Chrome, Firefox, Safari (IOS) and Firefox (IOS) but none of them work.
There seems to be no option to set a “Display name” in that combined “Username”/“Display name” field in the UI, if the user is already created, and exists in the database.
@alogwe, If I understood you correctly, your issue isn’t exactly the same one as the one started by OP (solved), but it’s the one that briught mne to this thread:
When creating a new user, admins can set their user name and display name
When editing an existing user, only one of these can be edited (the display name): when you click the pencil icon on the far right, there’s only a single box in the “username” space.
If, like me, you gave a user the same display name and user name, it’s not immediately obvious which one you’re editing there.
IMO this is a design flaw in the UI, as neumerous people have posted this problem across the internet - it should be made more obvious in some way that the user is not editable, while that edit box allows for changing the display name.
But if you try it, and save your changes successfully (press enter or hit arrow icon in edit field) you’ll notice that the display name changed, and the user name remained the same.
(As per this thread from 2017, it’s not possible to change an existing user’s username, because username is also Id in the database.)