I have this problem. No open K2 USER PAGE
ERROR 0 SQL=SELECT juser.*, k2user.group, k2group.name as groupname FROM zhlic_users as juser LEFT JOIN zhlic_k2_users as k2user ON juser.id=k2user.userID LEFT JOIN zhlic_k2_user_groups as k2group ON k2user.group=k2group.id LEFT JOIN zhlic_user_usergroup_map as `map` ON juser.id=map.user_id WHERE juser.id>0 GROUP BY juser.id ORDER BY juser.usertype asc LIMIT 0, 50

Which version of Joomla! and K2 are you using?

2.6.8

@Primapress Agenzia Stampa Nazionale

It looks like the issue occurs only when you sort the users list by the column "Joomla! Group" . Logout from administration and login again to reset the ordering filter. We will fix this as soon as possible.

And here is the fix if you want to apply it : github.com/joomlaworks/k2/commit/a5d32550696f76a004d7647949fb8f362fc32b46 .