Fatal error when editing a Personal Campaign Page in Joomla

Reproduced on Joomla demo as follows:
1. Login to front-end as user: alicefoo password: password
2. Go to "Your Dashboard" menu item (which is the Contact Dashboard)
3. Click link to Edit alice's PCP page
(takes you to: http://joomla.demo.civicrm.org/index.php?option=com_civicrm&task=civicrm/pcp/info&Itemid=105&action=update&reset=1&id=1&component=contribute)
4. Click SAVE

NOTE: If Joomla demo has been reset, you might have to do some setup again - mainly to set civi permission on Contact Dashboard so 'Registered' group has access.

NOTE:I also got the same fatal when trying to create and save a new PCP from the Contact Dashboard -> Create a Personal Campaign page link, when logged in as demo / demo.

— fatal error -----
LogicException: "Passed array does not specify a callable static method (cannot access private method JLoader::_autoload())"

#0 /var/www/drupal7.civicrm.org/svn/civicrm42/packages/IDS/vendors/htmlpurifier/HTMLPurifier/Bootstrap.php(98): spl_autoload_register((Array:2))
#1 /var/www/drupal7.civicrm.org/svn/civicrm42/packages/IDS/vendors/htmlpurifier/HTMLPurifier.autoload.php(11): HTMLPurifier_Bootstrap::registerAutoload()
#2 /var/www/drupal7.civicrm.org/svn/civicrm42/packages/IDS/vendors/htmlpurifier/HTMLPurifier.auto.php(9): require_once("/var/www/drupal7.civicrm.org/svn/civicrm42/packages/IDS/vendors/htmlpurifier/...")
#3 /var/www/drupal7.civicrm.org/svn/civicrm42/CRM/Utils/String.php(585): require_once("/var/www/drupal7.civicrm.org/svn/civicrm42/packages/IDS/vendors/htmlpurifier/...")
#4 /var/www/drupal7.civicrm.org/svn/civicrm42/CRM/PCP/Form/Campaign.php(211): CRM_Utils_String::purifyHTML("I'm on a mission to get all my friends and family to help support my favorite...")
#5 /var/www/drupal7.civicrm.org/svn/civicrm42/CRM/Core/Form.php(246): CRM_PCP_Form_Campaign->postProcess()
#6 /var/www/drupal7.civicrm.org/svn/civicrm42/CRM/Core/QuickForm/Action/Upload.php(151): CRM_Core_Form->mainProcess()
#7 /var/www/drupal7.civicrm.org/svn/civicrm42/CRM/Core/QuickForm/Action/Upload.php(128): CRM_Core_QuickForm_Action_Upload->realPerform(Object(CRM_PCP_Form_Campaign), "upload")
#8 /var/www/drupal7.civicrm.org/svn/civicrm42/packages/HTML/QuickForm/Controller.php(203): CRM_Core_QuickForm_Action_Upload->perform(Object(CRM_PCP_Form_Campaign), "upload")
#9 /var/www/drupal7.civicrm.org/svn/civicrm42/packages/HTML/QuickForm/Page.php(103): HTML_QuickForm_Controller->handle(Object(CRM_PCP_Form_Campaign), "upload")
#10 /var/www/drupal7.civicrm.org/svn/civicrm42/CRM/Core/Controller.php(289): HTML_QuickForm_Page->handle("upload")
#11 /var/www/drupal7.civicrm.org/svn/civicrm42/CRM/PCP/Page/PCPInfo.php(335): CRM_Core_Controller->run()
#12 /var/www/drupal7.civicrm.org/svn/civicrm42/CRM/Core/Invoke.php(220): CRM_PCP_Page_PCPInfo->run((Array:3), NULL)
#13 /var/www/drupal7.civicrm.org/svn/civicrm42/CRM/Core/Invoke.php(51): CRM_Core_Invoke::_invoke((Array:3))
#14 /var/www/joomla.demo.civicrm.org/public/components/com_civicrm/civicrm.php(78): CRM_Core_Invoke::invoke((Array:3))
#15 /var/www/joomla.demo.civicrm.org/public/components/com_civicrm/civicrm.php(18): civicrm_invoke()
#16 /var/www/joomla.demo.civicrm.org/public/libraries/joomla/application/component/helper.php(383): require_once("/var/www/joomla.demo.civicrm.org/public/components/com_civicrm/civicrm.php")
#17 /var/www/joomla.demo.civicrm.org/public/libraries/joomla/application/component/helper.php(351): JComponentHelper::executeComponent("/var/www/joomla.demo.civicrm.org/public/components/com_civicrm/civicrm.php")
#18 /var/www/joomla.demo.civicrm.org/public/includes/application.php(197): JComponentHelper::renderComponent("com_civicrm")
#19 /var/www/joomla.demo.civicrm.org/public/index.php(42): JSite->dispatch()
#20

{main}

Sorry but we are not able to provide this at the moment.
Passed array does not specify a callable static method (cannot access private method JLoader::_autoload())