This is what I found out so far:
- I have a second domain that I also manage on the Bing Webmaster site which has the same issue.
- And I don't have a third domain managed on the Bing Webmaster site. In general I have other domains. They are not affected.
- The content of one of my domains managed on the Bing Webmaster site is hosted on Bluehost the other on OpenShift.
- Only the landing page (main index.html) is affected. Other pages on the site are not affected.
- The added content seems not to be a part of the actual Bing index. It just shows up in the cache.
- My own page doesn't contain any code that might have a virus etc. (I am a web developer and I checked everything.)