I had very same problem as others before me. I have installed properly the mantis 1.2.3 and it is worked. Then I installed the Plugin Manager 0.4.0. It said everything is OK. Then when I want "Return to Plugin Manager" I got the error:
"Fatal error: Call to undefined function plugins_pluginmanager_get_plugin_list() in /path/docroot/plugins/pluginmanager/lang_inc.php on line 23"
I have done these things twice.
Whereas the earlier comments did not help me so I did a debug in the whole mantis.
Finally I found the source of the incident.
There is an undefined or an out of sight variable: $g_absolute_path
filter_api.php::52
require_once( $g_absolute_path . &
#039;config_filter_defaults_inc.php&
#039; );
Because did not help me to I have defined the variable in config (which has been created by mantis), I changed it:
require_once(dirname(dirname(__FILE__)).DIRECTORY_SEPARATOR.&
#039;config_filter_defaults_inc.php&
#039;
Then the error disappeared and I can use the mantis with plugin manager.