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I cant login to site administrator, invalid detail.
if that module not exist, please try to add new module from joomla module manager. and select JS Online Users.
please check it. at jomsocial 4.1.1, who is online is part of jomsocial members module.
here what I did:
1. goto joomla backend > extentions > modules, create new, select JS members
2. put title Jelenlévők, position sidebar-a, and set to display online members only