The memberlist did not display the column "Last active" and after investigating this I found that
$auth->acl_get('u_viewonline')
in memberlist.php
in lines 1833 and 1838 returned false
which was weird because the user role "Standard Features" showed in the "Profile" tab that the permission "Can view profiles, memberlist and online list" was set to "Yes", so it should have displayed the column in question.Some time later I was was desperate and checked all other tabs and found that in the "Misc" tab the setting "Can view hidden online users" was set to "Never". Switching this setting to "No" resolved the problem and the column "Last active" was displayed properly.
Now I am wondering why a "Never" setting for viewing hidden online users prohibits viewing the memberlist's column "Last active" since - still after some consideration - I can not imagine why this should be the case as these two setting seem to have nothing in common.
Is this a bug which I should report in the tracker or is this behaviour intended?