archive.php SCHEDULED TASKS error (no replies)

We have archive.php running every hour for around 700 sites. It usually runs fine without error. Except that every Monday we get errors like this:

SCHEDULED TASKS
Starting Scheduled tasks... 
ERROR: Got invalid response from API request: http://OURDOMAIN/index.php?module=API&method=CoreAdminHome.runScheduledTasks&format=csv&convertToUnicode=0&token_auth=OURTOKEN&trigger=archivephp. Response was '<div style='word-wrap: break-word; border: 3px solid red; padding:4px; width:70%; background-color:#FFFF96;'>
        <strong>There is an error. Please report the message (Piwik 1.12)
        and full backtrace in the <a href='?module=Proxy&action=redirect&url=http://forum.piwik.org' target='_blank'>Piwik forums</a> (please do a Search first as it might have been reported already!).<br /><br/>
        Notice:</strong> <i>Undefined variable: message</i> in <b>/var/www/html/piwik/plugins/PDFReports/PDFReports.php</b> on line <b>347</b>
<br /><br />Backtrace --&gt;<div style="font-family:Courier;font-size:10pt"><br />
#0  Piwik_ErrorHandler(...) called at [/var/www/html/piwik/plugins/PDFReports/PDFReports.php:347]<br />
#1  Piwik_PDFReports-&gt;sendReport(...) called at [:]<br />
#2  call_user_func_array(...) called at [/var/www/html/piwik/libs/Event/Dispatcher.php:284]<br />
#3  Event_Dispatcher-&gt;postNotification(...) called at [/var/www/html/piwik/core/PluginsManager.php:674]<br />
#4  Piwik_PostEvent(...) called at [/var/www/html/piwik/plugins/PDFReports/API.php:535]<br />
#5  Piwik_PDFReports_API-&gt;sendReport(...) called at [:]<br />
#6  call_user_func(...) called at [/var/www/html/piwik/core/TaskScheduler.php:157]<br />
#7  Piwik_TaskScheduler::executeTask(...) called at [/var/www/html/piwik/core/TaskScheduler.php:72]<br />
#8  Piwik_TaskScheduler::runTasks(...) called at [/var/www/html/piwik/plugins/CoreAdminHome/API.php:38]<br />
#9  Piwik_CoreAdminHome_API-&gt;runScheduledTasks(...) called at [:]<br />
#10  call_user_func_array(...) called at [/var/www/html/piwik/core/API/Proxy.php:188]<br />
#11  Piwik_API_Proxy-&gt;call(...) called at [/var/www/html/piwik/core/API/Request.php:145]<br />
#12  Piwik_API_Request-&gt;process(...) called at [/var/www/html/piwik/plugins/API/Controller.php:25]<br />
#13  Piwik_API_Controller-&gt;index(...) called at [:]<br />
#14  call_user_func_array(...) called at [/var/www/html/piwik/core/FrontController.php:125]<br />
#15  Piwik_FrontController-&gt;dispatch(...) called at [/var/www/html/piwik/index.php:47]</div><br />
 </pre></div><br />task,output
Piwik_PDFReports_API.sendReport_425,Time elapsed: 1.124s'

Is there a known issue with SCHEDULED TASKS phase of archiving? Is there a suggested fix?