Your configuration in the table is broken. It is a mix with a second old configuration. Please make following steps:
1. Create a database backup (a backup from the configuration table have you already)
2. Important! First go in backend to Kunena -> Configuration. You have to stay in the configuration during the next steps!
3. Open a second browser window and go in your database and open the table *_kunena_configuration
4. Click on edit at this table and empty the textfield (params)
5. Save it
6. Now go back into the first browser window and save the configuration
Then creates the configuration params in the database new. The first action in the backend is to save the configuration! If you navigate in the backend before you have saved the configuration, then you have all default settings again.
7. Empty the cache
8. Go in backend to Kunena -> Tools - Recount Statistics and let count all statistics new.
I hope, this removes the error.
I am getting the same error upon reply submission:
Argument 1 passed to KunenaForumMessage::attachEmailBody() must be an instance of Joomla\CMS\Mail\Mail, instance of JMail given, called in /home/inventri/public_html/libraries/kunena/forum/message/message.php on line 487
I tried the fix above - deleted the config in the database - re-saved in the back end. It didn't seem to fix the issue.
I've cleared every Cache I could think of. For some reason it still does this. I've pulled the php file and looked at line 487 and it has the proper CMS\Mail\Mail info in it.