Warning: Cannot modify header information – headers already sent by class-wp-que

@steindialog that seems to be a conflict with other plugin. Are the warning only showing when you activate it, or after activated you are always seeing it?

Hi Manuel,

I have added this in wp-config.php
define( ‘WP_DEBUG’, true );

and therefore I see it always if the plugin is activated.
If I de-activate the warning is gone.

BR; Alex

@steindialog tested on my side and it doesn’t work… seems a conflict with other plugin. Can You provide me a list of installed plugins?

Dear Manuel,

please find here a list of all 30 activated plugins:

Add-on WooCommerce MailPoet 3
Akismet Anti-Spam
Cloudflare Flexible SSL
Contact Form 7
Contact Form 7 Captcha
Contact Form 7 Skins
Duplicate Page
EWWW Image Optimizer
Flamingo
Font Awesome
Gutenberg
Health Check & Troubleshooting
Honeypot for Contact Form 7
Insert Headers and Footers
Jetpack von WordPress.com
MailPoet 3 (New)
Members
Product CSV Import Export (BASIC)
Redirection for Contact Form 7
Shortcodes Ultimate
Simple Google reCAPTCHA
Ultimate FAQ
WooCommerce
WooCommerce Services
WP Mail Logging
WP Mail SMTP
WP Staging
Yoast SEO

Those 21 are inactive plugins:

Automated DHL Express Shipping
CF7 DatePicker
Contact Form 7 Conditional Fields
Contact Form CFDB7
DHL for WooCommerce
Elementor
ElementsKit Lite
Facebook for WooCommerce
Germanized für WooCommerce
Glue for Yoast SEO & AMP
Google Fonts Typography
Google Tag Manager for WordPress
Hello Dolly
Insert HTML Snippet
Order / Coupon / Subscription Export Import Plugin for WooCommerce (BASIC)
Really Simple SSL
SSL Insecure Content Fixer
The GDPR Framework
WooCommerce Admin
Woody ad snippets (PHP snippets | Insert PHP)
Wordfence Security

Even if I switch the theme to “twentytwenty” the error notice is still alive:
Screenshot > https://www.screencast.com/t/G9iBMev3yqU

BR, Alex

• This reply was modified 4 years, 6 months ago by steindialog.
• This reply was modified 4 years, 6 months ago by steindialog.

Dear Manuel,

menawhile I could find the plugin which makes the issue.
It’s the “Add-on WooCommerce – MailPoet 3”

If I deactivate that, then those error messages are gone.

Unfortunately I get no response from their support with the tread so far.

Maybe you have an idea in – how to solve that.

BR, Alex

@steindialog in fact those warnings were not generated from our plugin.

I cannot help you with that plugin because I don’t know it. As far as I understand it seems to be an issue that the plugin is not getting an expected post?! Don’t know. You will probably have to look into the plugin code and try to understand why.

But as this is just warning/notice messages I would not give such importance… If the plugin is doing is job those messages will go away as soon as you disable the wp_debug…