@lavolp3 I should have thought a bit longer.
The reason is that the number for the league is deprecated. If you use the actual number, it works.
curl -H 'X-Response-Control: minified' -X GET http://api.football-data.org/v1/competitions/?season=2017
[{"id":444,"caption":"Campeonato Brasileiro da Série A","league":"BSA","year":"2017","currentMatchday":20,"numberOfMatchdays":38,"numberOfTeams":20,"numberOfGames":380,"lastUpdated":"2017-08-24T00:40:05Z"},
{"id":445,"caption":"Premier League 2017/18","league":"PL","year":"2017","currentMatchday":3,"numberOfMatchdays":38,"numberOfTeams":20,"numberOfGames":380,"lastUpdated":"2017-08-24T10:00:01Z"},
{"id":446,"caption":"Championship 2017/18","league":"ELC","year":"2017","currentMatchday":5,"numberOfMatchdays":46,"numberOfTeams":24,"numberOfGames":552,"lastUpdated":"2017-08-22T07:00:02Z"},
{"id":447,"caption":"League One 2017/18","league":"EL1","year":"2017","currentMatchday":4,"numberOfMatchdays":46,"numberOfTeams":24,"numberOfGames":552,"lastUpdated":"2017-08-22T04:00:06Z"},
{"id":448,"caption":"League Two 2017/18","league":"EL2","year":"2017","currentMatchday":4,"numberOfMatchdays":46,"numberOfTeams":24,"numberOfGames":552,"lastUpdated":"2017-08-22T04:00:06Z"},
{"id":449,"caption":"Eredivisie 2017/18","league":"DED","year":"2017","currentMatchday":3,"numberOfMatchdays":34,"numberOfTeams":18,"numberOfGames":306,"lastUpdated":"2017-08-23T04:00:08Z"},
{"id":450,"caption":"Ligue 1 2017/18","league":"FL1","year":"2017","currentMatchday":4,"numberOfMatchdays":38,"numberOfTeams":20,"numberOfGames":380,"lastUpdated":"2017-08-23T10:00:06Z"},
{"id":451,"caption":"Ligue 2 2017/18","league":"FL2","year":"2017","currentMatchday":5,"numberOfMatchdays":38,"numberOfTeams":20,"numberOfGames":380,"lastUpdated":"2017-08-24T07:00:10Z"},
{"id":452,"caption":"1. Bundesliga 2017/18","league":"BL1","year":"2017","currentMatchday":2,"numberOfMatchdays":34,"numberOfTeams":18,"numberOfGames":306,"lastUpdated":"2017-08-23T07:00:10Z"},
{"id":453,"caption":"2. Bundesliga 2017/18","league":"BL2","year":"2017","currentMatchday":4,"numberOfMatchdays":34,"numberOfTeams":18,"numberOfGames":306,"lastUpdated":"2017-08-24T07:00:15Z"},
[...]
For 1. Bundesliga 2017/2018 it’s 452