I've attached the Jenkins log file. This error occurs for any job that is a multi-configuration project that uses the "Sauce Labs WebDriver tests" axis for the configuration matrix. The build.xml file for the axis configurations will save, but the build.xml file for the base job will not causing the builds to fail to stick around in the build history.
Note: I reproduced this on a clean Jenkins instance where it was the minimal plugins past install and only the job used to reproduce this. This has also been happening on our main Jenkins instance stemming since at least Jenkins 2.60 that I can remember. Our main instance is currently running 2.110 and it's still happening. This has been around for multiple sauce ondemand plugin releases too.