To turn on debugging try the following things:
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1. If you can get to admininstration menu, go to Preferences, select 'Main', and find the section on debugging options. Choose "PHP Debugging"

2. If you can't get to the administration page, but have access to mysql, try the query "UPDATE xoops_config SET conf_value=1 WHERE conf_name = 'debug_mode'". Be sure to make sure "xoops_config" matches the name of the config table in your installation. (NOTE: This does not work with XOOPS 2.2, see 2a)

2a. In XOOPS 2.2, go to your site's root file "recovery.php" and set the debug settings through the interface here

3. If you can't get to the administration page, and don't want to mess with mysql, but don't mind messing with PHP, then edit the file "include/common.php". Near line 83, change "error_reporting(0);" to "error_reporting(E_ALL);".

4. If this still doesn't work, your server or hoster may have turned off debugging in "/etc/php.ini" (linux) or "WIN_DIRECTORY/php.ini" (windows). Make sure there is a line in there "display_errors On".

5. If you don't have access to change this file (e.g. on shared hosting), then if the server uses the "apache" webserver, you can create a file called ".htaccess" to override the settings locally. This file should contain the line: "php_flag display_errors on". The tricky part is WHERE to put this file. Look at the URL you are having problems with. Put the file in the corresponding directory. e.g. If it is "someplace.com/xoops/" or "someplace.com/xoops/index.php" then you need to put the file in the main XOOPS directory. If it is something like "someplace.com/xoops/modules/system/admin.php" then put it in "modules/system" subdirectory of your XOOPS installation. etc.