Adding an answer here since I had a similar problem.
It might not 100% the case in point, but there are more than one similarity.
I solved it by following this solution. Basically I stopped building the project and re-started building again right after.
In retrospect I now understand that in my case this happened:
- I set up the project on Circle CI with master as my default branch and “only build PRs” on)
- Everything was running normally
- I switched default branch to develop
- Soon after builds stopped because of this bug
Hopefully this helps someone.