In my case, we had Jenkins access GitLab through a reverse proxy - nginx. Gitlab was running inside a docker container. I could nicely configure then Jenkins job itself with the GitLab Branch Source Plugin, it even nicely auto-discovered the projects for the owner that I set. However, once it tried to do the "Scan GitLab Project Now", it would give me an error saying "Not found (404)" for the project. For debugging, I ran a `tcpdump -A port 80` in the GitLab container, showing an access path looking like this: ```sh https://my-gitlab.com/projects/my-group/my-project/... ``` Clearly, this didn't match the GitLab API URL for projects, which expects an ":id": [https://docs.gitlab.com/ee/api/projects.html](). It took a long while to figure out, that the API component of the Branch Source Plugin actually URL-encodes the "project path" as the id, i.e. the request URL is actually: ```sh https://my-gitlab.com/projects/my-group%2Fmy-project/... ``` **Turns out, you have to tell nginx to use the original URI as-is (and not automatically unwrap/decode the URI...):** ```sh Wrong: proxy_pass http://127.0.0.1:8929/ Correct: proxy_pass http://127.0.0.1:8929/$request_uri ``` More info in this StackOverflow thread: [https://stackoverflow.com/questions/20496963/avoid-nginx-decoding-query-parameters-on-proxy-pass-equivalent-to-allowencodeds]()