How to troubleshoot an error during GitLab pages upload?

I run my pages job and it passes however with the following message at the end

Uploading artifacts...
WARNING: public: no matching files                 
Uploading artifacts to coordinator... ok            id=14329074 responseStatus=201 Created token=V9gP9h2U
Job succeeded

The website appears not to be served. All the build steps succeeded without error. I did the build locally on my machine and verified it is correct. The website’s entry point is index.html (I guess that’s correct?).

How can I troubleshoot this problem? It would be nice if I could perform a “manual” job so I could check a few things on the build machine. Like that I don’t have to commit+push a new .gitlab-ci.yml all the time.

Any suggestions are highly appreciated! Thanks!

P.S.: I build the website using Sphinx if that is of importance.