QuietNoise

I tried to debug it with my humble abilities.

After clicking logout WP user is being logged out from WP account and from FB account (for whatever reason. I would prefer that I’m still logged in to FB as it is different website).
Anyway, after redirecting back from FB logout call to the website
$this->is_user_logged_in_facebook() returns true (line: 2069) which causes chain of other calls which at the end results in the mentioned error.

What exactly happens:
$this->login($redirect_url); is called and it checks:
if ($this->uid) { // which is true (but why if I’m logged out)
because of that
$this->get_facebook_user_data(); is called which causes the error.

The question is why $this->uid is set if I’m supposed to be logged out?

Furthermore, in line: 1976
$this->uid = $this->fcbk->getUser();
user id is returned. But at this particular moment I’m logged out from facebook.

Another strange behavior, not sure if related
If I’m logged out from WP it signs me back in after I sign in to Facebook. Is this desired behavior?

I’m not sure if I can spend more time on this one. Need deliver working solution.

Alright. Here is what I got so far:
I’ve changed the option “Url after logout” in your plugin settings. However it didn’t influence shape of the url of WordPress logout link (not the AWD FB logout link). It is still:
https://www.facebook.com/logout.php?next=https://mytestdomain.com/facebook-awd/logout?redirect_to=index.php&_wpnonce=b69d42fb25&access_token=AAAEv…

However, I noticed that it does change the logout button generated by your plugin.
The button name is now the “after logout url” I entered in plugin settings (before it was just the word “Logout”).
The button code is now:
<a href="https://www.facebook.com/logout.php?next=https://mytestdomain.com/facebook-awd/logout?redirect_to=https://mytestdomain.com/users/?action=login&_wpnonce=b69d42fb25&access_token=AAAEvn...">https://mytestdomain.com/my-board</a>

Please notice that redirect_to has value I entered in “Url after login” in plugin settings. And the anchor tag content is: https://mytestdomain.com/my-board which is url which I entered “Url after logout” plugin settings.

The error still appears no matter which logout link I press.

https://www.facebook.com/logout.php?next=http%3A%2F%2Fmytestdomain.com%2Ffacebook-awd%2Flogout%3Fredirect_to%3Dindex.php%26amp%3B_wpnonce%3Db69d42fb25&access_token=AAAEvnD4aoZAgBAGZCRiuSxKHxrrmuAZBZAN8GUm4PGAm28WZBxpEAdoXL0esv1ZCrkZC2y6my8en6xB6IJlaJXAOzdNySFc2AnxcezoO2or9wZDZD

What I learned so far is that error happens not during wp_logout process but after redirect.

It happens on all browsers I tried.
Don’t have a public URL yet am testing on local environment.

I nailed the line where the error comes from.
It’s in AWD_facebook.php line 1899.
My WP version: 3.4.2.

Call stack:
wp-content/plugins/facebook-awd/AWD_facebook.php.AWD_facebook->get_facebook_user_data:1899
wp-content/plugins/facebook-awd/AWD_facebook.php.AWD_facebook->login:2079
wp-content/plugins/facebook-awd/AWD_facebook.php.AWD_facebook->login_listener:2070
wp-content/plugins/facebook-awd/AWD_facebook.php.AWD_facebook->parse_request:2187
wp-includes/plugin.php.call_user_func_array:484
wp-includes/plugin.php.do_action_ref_array:484
wp-includes/query.php.WP_Query->parse_query:1674
wp-includes/query.php.WP_Query->get_posts:1920
wp-includes/query.php.WP_Query->query:2941
wp-includes/class-wp.php.WP->query_posts:460
wp-includes/class-wp.php.WP->main:524
wp-includes/functions.php.wp:775
wp-blog-header.php.require:14
index.php.{main}:19

Hi, got the same error. After you login with FB and then when you click on WP logout the error shows preventing any page to display.

The problem might be in the cookies handling. After deleting cookies related to the website (specifically PHPSESSID and fbsr_xxxx) the error disappears.

Edit:
It’s your plugin’s generated error so it’s hard to say from which line of code it is generated.
When enabling WP Debug the following PHP notice shows:
Notice: ob_end_flush() ref.outcontrol: failed to delete buffer zlib output compression. in /wp-includes/functions.php on line 2504 but I have no clue if it is relevant at all.

Same here. It is very suspicious.
You folks should try other solutions as this one is dangerous for your visitors.
There might be some other trap in it and you don’t know when it is gonna trigger.

I confirm several “Notice: Undefined index: ” errors scattered around the place when using the plugin.
It could be a good practice to check for indexes before using them. It is troublesome during development and whether I want it or not I have to fix these errors manually.

On activation it outputs error mentioned by OP.
On deactivating and subsequent activation it outputs some unexpected data.
On Advanced Settings page it throws:

Notice: Undefined index: template_email in /wp-content/plugins/wordpress-form-manager/pages/editsettingsadv.php on line 215
Notice: Undefined index: template_summary in /wp-content/plugins/wordpress-form-manager/pages/editsettingsadv.php on line 216

On resetting templates:
Notice: Undefined variable: template in /wp-content/plugins/wordpress-form-manager/template.php on line 152

It was throwing something on front end as well but I lost it. Will update once I get it again.

Edit:
Other errors I found in the log file. Some of them are SQL error. Not sure how much this impacts the plugin functionality.
PHP Notice: Undefined variable: charset_collage in /wp-content/plugins/wordpress-form-manager/db.php on line 378
PHP Notice: Undefined index: recaptcha_theme_custom in /wp-content/plugins/wordpress-form-manager/pages/editsettings.php on line 19
WordPress database error Multiple primary key defined for query ALTER TABLE wp_fm_forms ADD PRIMARY KEY (ID) made by activate_plugin, do_action(‘activate_wordpress-form-manager/wordpress-form-manager.php’), call_user_func_array, fm_install, fm_db_class->setupFormManager, dbDelta
WordPress database error Multiple primary key defined for query ALTER TABLE wp_fm_settings ADD PRIMARY KEY (setting_name) made by activate_plugin, do_action(‘activate_wordpress-form-manager/wordpress-form-manager.php’), call_user_func_array, fm_install, fm_db_class->setupFormManager, dbDelta
WordPress database error You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ‘set set INT DEFAULT ‘0’ NOT NULL’ at line 1 for query ALTER TABLE wp_fm_items CHANGE COLUMN set set INT DEFAULT ‘0’ NOT NULL made by activate_plugin, do_action(‘activate_wordpress-form-manager/wordpress-form-manager.php’), call_user_func_array, fm_install, fm_db_class->setupFormManager, dbDelta
WordPress database error Duplicate column name ‘index’ for query ALTER TABLE wp_fm_items ADD index INT DEFAULT ‘0’ NOT NULL made by activate_plugin, do_action(‘activate_wordpress-form-manager/wordpress-form-manager.php’), call_user_func_array, fm_install, fm_db_class->setupFormManager, dbDelta

FYI I didn’t create any forms yet.

WordPress 3.4.1
Plugin 1.6.37

I confirm the notices after plugin activation and in admin panel.
PHP Notice: Undefined index: task in /var/www/vhosts/samplesite/httpdocs/wp-content/plugins/download-manager/wpdm-server-file-browser.php on line 30
PHP Notice: Undefined index: task in /var/www/vhosts/samplesite/httpdocs/wp-content/plugins/download-manager/wpdm-server-file-browser.php on line 4

Also some other notices scattered around in admin panel while trying to use the plugin

WordPress 3.4.1.
Plugin 2.2.8