I now have found the conflict. Its the security plugin that stops email address encoder. It do not cause any problem for any of my other plugins.
I use the plugin “All In One WP Security” with normal file protection. I do not understand why email address encoder is blocked and no other plugin. But when I turn of the security plugin I can fully use email address encoder features.
Below is what they write:
“Your WordPress file and folder permission settings govern the accessability and read/write privileges of the files and folders which make up your WP installation.
Your WP installation already comes with reasonably secure file permission settings for the filesystem.
However, sometimes people or other plugins modify the various permission settings of certain core WP folders or files such that they end up making their site less secure because they chose the wrong permission values.
This feature will scan the critical WP core folders and files and will highlight any permission settings which are insecure.”