If you create a promotion process with and invalid character in the name (such as a /) you will get a page saying "Error : Could not create" once you save the job. The plugin will still create the directory under the promotions directory under the job. Now, no promotions will show on the job and attempts to create an subsequent promotion processes will fail.
The promoted build plugin should validate the promotion name and disallow invalid filesystem path characters.
Workaround: Manually delete the invalid promotion from <HUDSON_HOME>/.hudson/jobs/<JOB_NAME>/promotions