This error can be caused by several factors but in the vast majority of cases it is due to a theme/plugin conflict.
To find out what is causing this error please first change your theme to the default theme.
If after changing to the default WordPress theme the error is still showing, please deactivate all other plugins and activate them one by one.
If after checking all the plugins and the error still does not go away, it may be due to a WordPress memory issue and will require you to increase the memory of your site.
If the error still exists after doing all of this, please get in touch via email and we will look into what is causing the error.