I found an error,
in Dashboard when you the flag they send you to the page im-thread-flag/2 (ERROR 404)

@mixradu
Thanks for reporting, it is bug in plugin.

Please go to file:
oc-content/plugins/instant_messenger/index.php

Find these codes:
im-thread-page/([0-9]+)/
im-thread-flag/([0-9]+)/
im-thread-notify/([0-9]+)/

Replace with:
im-thread-page/([0-9]+)
im-thread-flag/([0-9]+)
im-thread-notify/([0-9]+)

Also there is one more bug.
In same file find line:
osc_add_route('im-thread-notify', 'im-thread-notify/([0-9]+)', 'im-thread-flag/{thread-notify-id}', osc_plugin_folder(__FILE__).'user/threads.php', true);

and replace with:
osc_add_route('im-thread-notify', 'im-thread-notify/([0-9]+)', 'im-thread-notify/{thread-notify-id}', osc_plugin_folder(__FILE__).'user/threads.php', true);