Where in the US-German DTA did you find an exemption for German rental income?  That's not how the article is read, which means the returns you prepared in the past were, prima facie, incorrect.  You may like to re-read the article.

As for the code, I didn't realize it would actually show up as a rejection code because it is actually an error code for notices, which means "We can’t allow your tax treaty exemption. The treaty you claimed is not a valid tax treaty."