No data for servers using the unix agent for a couple of weeks now. Have Observium build 3397 installed. Just had a chance to do some troubleshooting...

Running poller update on a specific server in debug mode reveals the apparent problem:

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including: includes/polling/unix_agent.inc.php
PHP Warning: include(includes/polling/unix_agent.inc.php): failed to open stream: No such file or directory in /opt/observium/includes/polling/functions.inc.php on line 192

Warning: include(includes/polling/unix_agent.inc.php): failed to open stream: No such file or directory in /opt/observium/includes/polling/functions.inc.php on line 192
PHP Warning: include(): Failed opening 'includes/polling/unix_agent.inc.php' for inclusion (include_path='.:/usr/share/pear:/usr/share/php') in /opt/observium/includes/polling/functions.inc.php on line 192

Warning: include(): Failed opening 'includes/polling/unix_agent.inc.php' for inclusion (include_path='.:/usr/share/pear:/usr/share/php') in /opt/observium/includes/polling/functions.inc.php on line 192

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Fixed (worked around) by running:

ln -s /opt/observium/includes/polling/unix-agent.inc.php /opt/observium/includes/polling/unix_agent.inc.php