Tag archive can't find my posts

Like most blogs, my blog show the related tags with every post. When I click the tag, it is supposed to give me an overview of all posts using that tag. However, my tag archives only display the following message:

Not Found

Apologies, but no results were found for the requested archive. Perhaps searching will help find a related post.

An example:
https://internetbbq.net/portfolio/fotografie/2011/konigsleiten09 (post)
https://internetbbq.net/project/teeest (tag archive).

Does anyone have any advice on how to resolve this issue?

Thanks in advance,

Segia