I have not seen this before. But behind the scenes the workflow engine for 2010 and 2013 workflows are different. 2010 workflows are saved in the database with the site and everything happens inside SharePoint. 2013 however uses an external system to process and run workflows. It's possible there is a problem with one or more of the components required to run 2013 workflows (workflow client / server pieces, workflow manager, or even databases).
you can't really troubleshoot this from the site interface. You need to be an admin and check the server logs and the SharePoint logs (ULS) to get the proper error messages that may be preventing you from saving the workflow.