Hello, since this is the first result from google search and probably others are also redirected here:
After spending ~2 hours on this problem (great error handing, just throw stack trace at the user without any hints about what possibly went wrong 
) I found this github issue: From Jenkins HTTP 404/Not Found · Issue #7122 · go-gitea/gitea · GitHub
Turns out you have to specify the “owner” field in folder configuration. It must be the name of organisation you want to pull projects of.