Error found : Start tag expected, '<' not found in must_have_module_list.xml file during backoffice login
Two solutions, either one will solve the problem.
1/. rename the must_have_modules_list.xml to must_have_modules_list.xml.bak
/prestashop folder/config/xml/must_have_modules_list.xml
2/. remove the cache files in /var/cache/*