include/classes/sysmaps/CCanvas.php
line 71, 72
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imagestring($this->canvas, 0, $this->width - 120, $this->height - 12, $date, $this->getColor('gray'));
imagestringup($this->canvas, 0, $this->width - 10, $this->height - 50, ZABBIX_HOMEPAGE, $this->getColor('gray'));
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Second parameter (font) shoudn't be zero for both lines.

As php manual states from http://www.php.net/manual/en/function.imagestring.php

bool imagestring ( resource $image , int $font , int $x , int $y , string $string , int $color )

font
Can be 1, 2, 3, 4, 5 for built-in fonts in latin2 encoding (where higher numbers corresponding to larger fonts) or any of your own font identifiers registered with imageloadfont().

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It works with native PHP, but does not with HHVM (http://hhvm.com/)

So just replace 0 -> 1