How to disable a plugin manually?

[polyfragmented]

Not sure what's causing it. Try renaming plugins.xml to plugins-old.xml and renaming the plugins directory to /plugins-old/

No luck there. After renaming the plugin folder and plugins.xml:

Notice: Undefined variable: pluginfiles in /www/htdocs/foo/admin/inc/plugin_functions.php on line 110

Warning: Invalid argument supplied for foreach() in /www/htdocs/foo/admin/inc/plugin_functions.php on line 110

Warning: require_once(/www/htdocs/foo/plugins/i18n_navigation.php) [function.require-once]: failed to open stream: No such file or directory in /www/htdocs/foo/admin/inc/plugin_functions.php on line 43

Fatal error: require_once() [function.require]: Failed opening required '/www/htdocs/foo/plugins/i18n_navigation.php' (include_path='.:/usr/share/php:..') in /www/htdocs/foo/admin/inc/plugin_functions.php on line 43

After only renaming the plugins.xml file:

Warning: file_get_contents(/www/htdocs/foo/data/other/user.xml) [function.file-get-contents]: failed to open stream: No such file or directory in /www/htdocs/foo/admin/inc/basic.php on line 218

Notice: Trying to get property of non-object in /www/htdocs/foo/plugins/p01-contact_gs.php on line 69

Now this after deleting the user.xml file the contact form plugin wants to have (which I just created as an empty file to see if it might help):

Warning: file_get_contents(/www/htdocs/foo/data/other/user.xml) [function.file-get-contents]: failed to open stream: No such file or directory in /www/htdocs/foo/admin/inc/basic.php on line 218

Notice: Trying to get property of non-object in /www/htdocs/foo/plugins/p01-contact_gs.php on line 69

Warning: Cannot modify header information - headers already sent by (output started at /www/htdocs/foo/admin/inc/basic.php:218) in /www/htdocs/foo/admin/inc/cookie_functions.php on line 30

Warning: Cannot modify header information - headers already sent by (output started at /www/htdocs/foo/admin/inc/basic.php:218) in /www/htdocs/foo/admin/inc/login_functions.php on line 81
Error: Headers already sent in /www/htdocs/foo/admin/inc/basic.php on line 218 If your browser does not redirect you, click here