excuse: I'm a super-beginner of OctoberCMS and ProBlog, so I'm afraid I miss some important things.

1. Enable [Settings] -> [ProBlog] -> [Editor] -> [Enable Markdown Editor / Preview]
2. Create new post with its content is written in markdown
3. Click [Create and Close] button
4. Select the newly created post to edit it
5. Do something like changing Excerpt or adding Tags, and click [Update] gives the error: "Class 'DOMDocument' not found" on line 465 of /home/***/vendor/erusev/parsedown-extra/ParsedownExtra.php

I think the reason why the error is given is that the [Content] tab contains HTML converted version of the post, instead original Markdown version. Is this intended behavior?

I found that php-xml extension is not installed. After installing the extension, the error goes away. Sorry I didn't noticed that.

Anyway, still [Content] tab contains HTML converted version. Is this intended? (Should I have to keep the original Markdown version somewhere else if I want to edit the content?)

I should look into this. Can you try commenting out line 139 of /plugins/radiantweb/problog/models/post.php and then re-edit in markdown and save and let me know if that resolves your issue?

It resolves my issue. After editing Post.php, [Content] tab shows Markdown version. ProBlog Post on the site renders as HTML version. Looks like no problem at all. Thank you!

Edited: /plugins/radiantweb/problog/models/Post.php

     public function afterFetch()
     {
         if (ProblogSettingsModel::get('markdownMode', true)) {
-             $this->content_markdown = $this->content;
+#            $this->content_markdown = $this->content;
         }
     }