Error message

Try switching to the default theme (Twenty Seventeen) and deactivating all other plugins.

Thank you very much.
1, switching is no option. Contact form 7 worked perfectly with the present theme and I would have to rebuild my site.
2. Deactivating plugins is the first thing I do when something does not work anymore

For you information: the error message is displayed but the form does what it is supposed to do.

Kind regards,
Robert

switching is no option. Contact form 7 worked perfectly with the present theme and I would have to rebuild my site.

This is a technique commonly used in WordPress to find what is actually causing a particular problem.

Switch temporarily (nothing will be lost when you switch back) to a WP default theme & disable all plugins other than Contact Form 7 to see if the problem goes away.

If it does you can switch back to your theme & re-activate each plugin one by one to see when the problem occurs.

id eigenschap van een WPCF7_ContactForm object is niet langer meer beschikbaar. Gebruik in plaats daarvan de methode id(). in /home/keesdnm53/domains/petebogsbigband.nl/public_html/wp-content/plugins/contact-form-7/includes/contact-form.php on line 162

I don’t speak Dutch but this looks like your CF7 plugin install may not have updated correctly.

Delete the plugin folder at wp-content/plugins/contact-form-7/ using FTP (don’t use Delete option on Plugins page as this may erase all CF7 forms) and install the plugin again.

Hi Neil,
your last remark did the trick.
Everything is up-and-running again!
Thanks!!
kind regards,
Robert

Hi, I found out that the problemmaker is Contact Form 7 Cost Calculator – Price Calculation. Enabling this extension caused the problem to return.
kind regards,
Robert

I found out that the problemmaker is Contact Form 7 Cost Calculator – Price Calculation. Enabling this extension caused the problem to return.

It’s often helpful for others if you can report the problem in the support forum for that plugin.