Here are the known causes of blank or 'white' pages:
1. You did not upload the vB files correctly. Reupload the vB non-image files and make sure you upload these in ASCII format and that you overwrite the ones on the server. Make sure you upload the Admin CP files to the admincp directory specified in your config.php file. Then, if you can access the Admin CP, run 'Suspect File Versions' in Diagnostics to make sure you have all the original files for your version:
Admin CP -> Maintenance -> Diagnostics -> Suspect File Versions
Do any show as 'File does not contain expected contents', 'version mismatch' or missing? If so, you need to reupload the original vB non-image files. Make sure you upload these in ASCII format and overwrite the ones on the server.
2. You have extra space or lines in your config.php file. Make sure there is no whitespace or extra lines in config.php either before the <?php or after the ?>. [Note: Beginning with 3.6.3 the trailing ?> was removed.]
3. If this is happening on the forum home page only, then you may have an empty index.html or index.htm file in that directory. Delete it.
4. You have a bad plugin installed. To disable the plugin system, edit config.php and add this line right under <?php
Note: If you are running vBSEO or other add-ons that use .htaccess rewrite, you will need to remove those changes as well.
5. The servername setting in config.php is wrong. Doublecheck this setting. 99% of the time, 'localhost' is correct:
$config['MasterServer']['servername'] = 'localhost';
6. Your PHP has magic_quotes_sybase turned on. You have to turn this off. On *nix systems you can do this by creating an .htaccess file with this content and placing it in your main forum directory:
php_flag magic_quotes_sybase 0
7. [For multiple white pages] You have added code to your header, headinclude or phpinclude templates that is no longer functional.
[For white pages in a select area] You have added code to one of your templates that is causing this problem.
The quickest way to find out if a custom template is at fault is to create a new style with no parent style and try that:
Admin CP -> Styles & Templates -> Style Manager -> Add New Style
8. You have a corrupted template. Repairing the template table may help:
REPAIR TABLE template;
9. You have GZIP enabled. Try turning GZIP off here:
Admin CP -> vBulletin Options -> Cookies and HTTP Header Options -> GZIP HTML Output -> No
Or by running these queries in the SQL tab in phpMyAdmin:
UPDATE setting SET value = '0' WHERE varname = 'gzipoutput';
UPDATE datastore SET data=REPLACE(data,'s:10:"gzipoutput";i:1;','s:10:"gzipoutput";i:0;') WHERE title='options';
You can also edit config.php and add this right under the <?php line to disable GZIP:
Sometimes this problem is caused when your server is already using GZIP and by turning this on in vB you were double compressing. This causes problems with some pages but not others. It also happens to some people and not others.
10. Sometimes this can also be caused when PHP has the 'display_errors' function turned off. So instead of displaying the actual error so you can see what is wrong, you get a blank page. Look at your phpinfo page and if 'display_errors' is Off or '0', then add this line to your includes/config.php file right under <?php
11. Check your phpinfo page to see if suhosin is installed as a module. If it is, this could be the cause of this problem. To fix this, add or edit an .htaccess file in your root forum directory and add these lines to it:
php_flag suhosin.cookie.encrypt Off
php_value suhosin.request.max_vars 2048
php_value suhosin.post.max_vars 2048
12. This can be caused by a bug in PHP 5.2.5:
13. This can also be caused by a memory_limit setting in php.ini that causes the server to time out before displaying the page. Edit config.php and add this right under the <?php line:
14. If the script producing this problem is showgroups.php, then you may have too many usergroups for this. Make sure this is set to 'No' for any groups except Admins and Mods:
Admin CP -> Usergroups -> Usergroup Manager -> Edit Usergroup -> Viewable on Show Groups
15. Check the file and directory permissions. Although this can differ by server, in general the directories should be chmod'd to 755 (-rwxr-xr-x) and files to 644 (-rw-r--r--). If any are set to 777 (-rwxrwxrwx) then this could result in blank pages.
16. Check your .htaccess file for any rewrite rules that may be effecting the page(s) you are having the issue with. If your problem is only in one particular directory, you may need to exclude the rewrite rules from working in that directory.
17. There is a bug in PHP 5.3.5 that can cause white pages during search and possibly other functions. The link to this bug is here: http://bugs.php.net/bug.php?id=51425
Further info is here: http://www.vbulletin.com/forum/showthread.php/371767-New-post-search-returning-blank-white-pages-%28no-errors-indicated%29?p=2108112#post2108112