It sounds like your upgrade to v1.3.6 didn't happen properly. You either missed some database updates, or missed updating some PHP files. This "symptom" may be an indication of one of several problems.
Nevertheless, try this:
1. Run this SQL query either in phpmyadmin or the sql patch tool:
Code:
UPDATE configuration SET set_function='zen_cfg_pull_down_htmleditors(' WHERE configuration_key='HTML_EDITOR_PREFERENCE';
2. Edit admin/includes/functions/general.php
around line 964 you should have:
Code:
function zen_cfg_pull_down_htmleditors($html_editor, $key = '') {
global $editors_list;
$name = (($key) ? 'configuration[' . $key . ']' : 'configuration_value');
$editors_pulldown = array();
foreach($editors_list as $key=>$value) {
$editors_pulldown[] = array('id' => $key, 'text' => $value['desc']);
}
return zen_draw_pull_down_menu($name, $editors_pulldown, $html_editor);
}
Make sure that function definition exists in that file.
If you've installed addons/contributions that alter this file you'll need to manually review all the changes those contributions have made.