I started to think that the problem with clearing the draft after sending the reply is due to some old files left before the update. Because today I noticed that even the view attachment button doesn't work because of the old .htaccess file I forgot to delete. Actually allowing access is not a good idea but since attachments are renamed, I don't really care.
So If anyone sees "There was an error loading the document." message when they want to view an image preview, they have to erase or rename the .htaccess file that blocks direct access to the attachments folder. (It is located at content/attachments)
PS: About the problems should I erase everything and install it from scratch? I assume every setting is saved on my DB and it is not related to these issues. I don't really care attachments but they are also easy to back-up.
I want to do this because that draft is really a pain in the ass. I have to erase it all the time )