Strange behavior when going into archives

My blog is showing very strange behavior. I do have a 3 column setup. Calendar and archive links on the right, text in the middle and links and categories on the left. The left column also includes Google adsense ads. And here is the problem – when going into the archives and selecting different months the ads disappear and the actual content of my 404.shtml file is being displayed (including the redirect back to the home page. I also have Google Adsense ads within the text – the same thing happens to these ads. It’s hard to explain. Please take a look at

https://www.webhostingreport.net/blog/ and then go into the January archive.

Any idea of what is going on and how I can fix it?

Thanks for any help.

Christoph