I got an error: Whoops, GitLab is currently unavailable. 503
https://gitlab.com/cn-services-sandbox/micro-math/micro-math-base-operation/-/branches
Ok. It looks that it is related to
Error fetching refs is shown when trying to select the source project for a merge request
I made some push force and it caused this problem.
I add new commits to master and branch and it solved the problem.