In your ACP at /admin/settings/tags increase the maximum tag count, it seems that feed item has more than 5 tags.
@vstoykov are you looking for something with a WYSIWYG editor, or what? A simple plugin with BBcode parsing is easy. One that also adds a new composer like nodebb-plugin-bbcodes is more difficult.
There is nodebb-plugin-redactor which you can use if what you're looking for a WYSIWYG editor.