Did you have HTML Editor 2 installed before?
Use a database admin tool like phpMyAdmin and delete all tables related to the "KFM"-filemanager.
Be sure to make backups of these tables before deleting them, in case something goes wrong.
All the tables have a naming convention like "kfm_something_somthingelse".
Then apply the new "HTML Editor 3" to your form page. Preview your form page in the browser. When using the filemanager for the first time, you will be asked to reload the page to have tha database refreshed. At that point all the necessary "kfm" database tables will be created and everything should now work.