[Mon Apr 20 17:07:21 2015] [error] [client a.b.c.d] PHP Fatal error: Call to a member function SetFormat() on a non-object in /var/www/mantis/plugins/MantisGraph/core/graph_api.php on line 338, referer: https://helpdesk.domain.com/plugin.php?page=MantisGraph/summary_graph_imp_status.php
Is there anywhere on your file system the file verdana.ttf?
If not, you have to install some Microsoft fonts, e.g, depending on your distribution something like
sudo apt-get install msttcorefonts
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