This question is an exact duplicate of:
Possible bug (probably quickly fixed?): I get "page not found" pages when clicking any tag in the Stack Exchange search results.
Click on a tag of any answers it displays, example: [python]
It leads to: https://stackexchange.com/questions/tagged/python
And that page is displaying "Page Not Found" instead of a list of Python tagged questions and answers.
The root of the problem seems to be that the URL is not constructed with the question's site but with the search result (stackexchange.com) site instead, lacking the tags.
For example, https://stackexchange.com/search?q=python leads you to a result page, that shows today amongst its results the following question: "Why is a simulation of a probability experiment off by a factor of 10?"
If we click that title, we go on the math.stackexchange.com's site, (Why is a simulation of a probability experiment off by a factor of 10?), and there, clicking on "Python" rightfully leads to https://math.stackexchange.com/questions/tagged/python
However if we directly click the Python tag on the search result page on stackexchange.com, it tries do display: https://stackexchange.com/questions/tagged/python (with no
math.), and that one displays the page not found error.
Lastly, if the same page is displayed on, for example, the "Hot Questions" section: https://stackexchange.com/, clicking directly its "python" tag would lead to the proper site (math.stackoverflow.com) and not stay on "stackexchange.com".
I.e., direct clicking a tag is working properly on the front page of stackexchange.com, but not working properly on its search result pages.