Feed broken – custom theme validation error

Hallo from Germany :),

the feed ratgeber.advoneo-development.de/blog/feed/ on my site ratgeber.advoneo-development.de is broken.

I already found out that it has something to do with my custom theme, because the feed works when I activate the wordpress standard theme.

The validation Error I get with feedvalidator.org is:

• Server returned HTTP Error 500: Internal Server Error
• HTTP error with content that looks like a feed
• line 1, column 1681: Unexpected Text
  … n Raten zusammenkommen.]]></description> ? <content:encoded><![CDATA[<p> …
• line 1, column 1887: XML parsing error: <unknown>:1:1901: no element found
  … dann ein Punkt erreicht, an […]]]></description>

The <description> -Tag seems to cause a problem. But it’s the standard wordpress excerp that I use.

Some time ago, the feed showed at least the latest post (although I defined a number of 15 posts in WP). But now it doesn’t work at all.

I checked all the php files in my theme, but there weren’t any blank spaces and the error I get is different than the common declaration problem. That’s why Plugins for fixing the feed don’t work for me either.

My Plugins are: All In One SEO Pack, Eggplant 301 Redirects, Google XML Sitemaps, iThemes Security, UpdraftPlus – Backup/Restore, W3 Total Cache, WP Smush

I don’t know how to identify the error ??
I really hope someone can help me.