[Lynne]

Have you tried using a default style? Or tried disabling your plugins via the config.php file?

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To create a default style, create a new style with no parent:

• Styles & Templates > Style Manager > Add New Style
• Parent Style: No Parent Style
• Title: Default vBulletin
• Allow User Selection: Yes
• Save

Set this as the default in AdminCP > Settings > Options > Styles & Languages > Set Default Style. Then go to Maintenance > Clear System Cache. Then try editing a post and see if you still have the same problem.

If so, try disabling your modifications/plugins and see if you still have this problem.
Note: To temporarily disable the plugin system, edit includes/config.php and add this line right under <?php

PHP Code:

define('DISABLE_HOOKS', true);