libraries/cms/installer/installer.php
Not OK
Install the Joomla 1.5 legacy API for the getinstance() to return a reference.
Actions:
Replace "function getInstance()" by "function &getInstance()"
This allow get the reference of the instance created instead of a copy.

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Jms Multi Sites
Version : 1.3.41 - FULL (Latest available: 1.3.41)
Patches definition version 1.3.37 (Latest available: 1.3.37)
Files Status
administrator/components/com_multisites/multisites.xml
OK
installation
OK
administrator/defines.php
OK
defines.php
OK
includes/defines_multisites.php
OK
includes/multisites.php
OK
installation/application/defines.php
OK
configuration.php
OK
administrator/components/com_config/model/application.php
OK
administrator/components/com_menus/models/menutypes.php
OK
libraries/cms/installer/installer.php
Not OK
Install the Joomla 1.5 legacy API for the getinstance() to return a reference.
Actions:
Replace "function getInstance()" by "function &getInstance()"
This allow get the reference of the instance created instead of a copy.
libraries/joomla/database/database.php
OK
libraries/joomla/filesystem/path.php
OK
libraries/joomla/session/session.php
OK
libraries/joomla/user/user.php
OK
libraries/legacy/view/legacy.php
OK
components/com_content/helpers/route.php
OK
components/com_content/models/articles.php
OK
Notes:
Some files needs to be patched.
Press the 'install' button to update your system.

Fix released in JMS 1.3.42 the day after that Joomla released their 3.4.0 version.
See change history at
www.jms2win.com/en/faq/change-history-13x

As usual, it is recommended to perform the update of all the extension (not only JMS) with the latest version before doing any Joomla upgrade