Thank you for your reply.

krileon wrote: com_cpanel is loading on every page or at least on CBs pages. To do this is must have a Joomla system plugin or a CB plugin (if on just CB pages). Disable it and it should resolve the error.

Do you mean "a plugin" is com_cpanel? I think "com_cpanel" is joomla default plugin controlling back end panel so I guess I should not disable it.
Or do you mean "a plugin" is something else which is affecting "com_cpanel"'s normal activity?

Don't understand what you mean by this. CB userlists are created within CB > Userlist Management. To manage users from backend (edit, delete, etc..) see CB > User Management.

Yes, "CB userlists " you mentioned is exactly the page got blank.

Thank you for your reply.

krileon wrote:

Please enable debug mode and maximum error reporting within Joomla global configuration. Once done navigate to CB > Userlist Management and review any on-screen errors. Please see our Support > FAQ for further blank page debugging.

I enabled maximum error reporting and check "CB > Userlist Management" but no message was shown, same blank page.

I checked server error log and find 3 related error reports as follows;

PHP Fatal error: require_once() [<a href='function.require'>function.require</a>]: Failed opening required '' (include_path='.:<span>/</span>usr<span>/</span>share/pear:<span>/</span>usr<span>/</span>share/php') in <span>/</span>var<span>/</span>www/vhosts/mydomain/httpdocs/administrator/components/com_cpanel/admin.cpanel.php on line 17, referer: mydomain/administrator/index.php?option=com_comprofiler&task=showusers


PHP Notice: Use of undefined constant _UE_ENABLESPOOFCHECK - assumed '_UE_ENABLESPOOFCHECK' in <span>/</span>var<span>/</span>www/vhosts/mydomain/httpdocs/administrator/components/com_comprofiler/admin.comprofiler.html.php on line 1900, referer: mydomain/administrator/index.php?option=com_comprofiler&task=showusers

PHP Notice: Use of undefined constant _UE_ENABLESPOOFCHECK_DESC - assumed '_UE_ENABLESPOOFCHECK_DESC' in <span>/</span>var<span>/</span>www/vhosts/mydomain/httpdocs/administrator/components/com_comprofiler/admin.comprofiler.html.php on line 1902, referer: mydomain/administrator/index.php?option=com_comprofiler&task=showusers

It was solved. Thank you very much for your continues and effective support.

According to your advise below,

Please review your language plugin and field titles/descriptions to ensure if you're using HTML that it's properly closed.

I found the field title (textarea) including HTML tag for which change paragraph between title and textarea;

"title&</label></td></tr></table><table><tr><td><label>"

This tag worked some fields without causing UserList blank, like delimiter, facebook_social_plugin, but not worked for avatar and text area field.

Anyway thank you again for your best advise.

Please note your install is also out of date. Please upgrade to Joomla 1.5.26 (current 1.5.15) and CB 1.8 (current 1.7.1).

Thank you for your advise. I understood CB upgrade is done by just installing new version. Is CB1.8 worked in PHP 5.1.x environment?