Can you create a blank page with this on it:
<?php
phpinfo();
?>
And see if your php has webp support in the section labeled 'gd'?
If it doesn't, then you would have to talk to your hosting provider to get a php version with webp support in the GD library. It has to be compiled in when php is installed, so it isn't easy to add yourself.
If you give me FTP access and a url where I can play with the editor to see the <br /> and <image /> tags being added, then I can take a look and correct it in the code. I'm not sure the cause or best solution without looking at what you have. I may be able to update the ckeditor version, or I may be able to change a configuration setting. Worst case scenario I may have to go into the source code and fix it manually.
