The Dev REST API has multiple endpoints that can take region entry keys either as path variables or as query parameters. If the key contains a '/' then it can cause problems with parsing the URL. The '.' character also causes problems.
The operations and endpoints that do not allow '.' and '/' in region keys are:
GET, PUT, and DELETE on /v1/{region}/{keys} POST on /v1/{region}?key={key}
This is a known issue which has been reported as GEODE-7938.
A fix is currently being worked on. The suggested workaround is not to use '.' and '/' characters in region keys.