Ok, the RSS, RSS2 and Atom feeds should now work again. Previously (to be precise right after I enabled then markdown plugin) they simply included unformated content which looked ugly at best and was lacking some semantic at worst ;-)
Thanks to this topic in the WordPress support forum I could fix this small problem and I will apply these fixes on gamerslog.com tomorrow (after getting some sleep).
To the posters involved in solving this problem: Thank you very much :-) You saved me from reading the RSS specification :-)
Do you want to give me feedback about this article in private? Please send it to comments@zerokspot.com.
Alternatively, this website also supports Webmentions. If you write a post on a blog that supports this technique, I should get notified about your link 🙂