The error message is reported if the nextBuildNumber is lower than the current build number:

hudson.model.Executor#run: Unexpected executor death
java.lang.IllegalStateException: cannot create a build with number NUMBER since that (or higher) is already in use among [NUMBER]

Unfortunately this does not report the actual project which is hitting the issue, so it is difficult to determine which project(s) need to be fixed.

You can easily reproduce this problem by triggering 3 builds and manually setting the nextBuildNumber file to be 1. Triggering a new build will kill the build executor thread.