@struth Have you, as admin, added some of the users? I ask because the default sorting on the lists is by “last active”. If a member never logged in (such as a member created by admin) then (s)he would not show up in the list by default. Change the sorting to Alphabetical and see if your users show up.
Since activating twentyfourteen resolved your issue then it is an issue with your original theme. You may want to check out that themes functions.php and see if there is maybe a function that hooked into the members-loop.php but then again it may be something else all together.
Have you altered the members-loop.php in any way directly or via a function that hooks into it?
If not have you tried activating different themes such as twentyfourteen to see if the issue persists?
@struth active 5 months ago