Do not use "/" character inside GeoServer Layers' names.
Why? GeoServer stores its data structure as folders and files. When creating Layer containing "/" character inside its name, it will be interpreted as folder name. During folder creation, all "/" characters will be treated as folder separators and nested folders structure instead of single folder will be created. It will make such Layer not working after GeoServer service restart.
As an example, try to create folder named
long / folder / name
by executing following command:
mkdir './long / folder / name/' -p
It will create tree of folders instead of single folder.
More details regarding GeoServer data directory structure at:
Structure of the data directory