There was a known issue with precomputed lighting builds getting stuck in 4.9. This was resolved with 4.10.
The ping coordinator is a normal log output that happens and will show this when swarm coordinator is not used. It’s not relevant to the lighting build getting stuck though.
Unfortunately, there are too many factors that could cause the lighting to build to get stuck to suggest something simple. For this reason it’s better to update to 4.10.