Text editor is formating the text badly and creates a lot of blank spaces between paragraphs

Every time an article from our forum/knowledge base is edited, a lot of <br>'s are created between each paragraph. The user needs to manually delete them and sometimes it is not possible to delete the spaces under pictures. I must manually delete the spaces (<br> tags) in the HTML version of the editor, which is very time-consuming.
It looks that this issue occurs in all our old articles, not the ones I was creating today. Please help.