According to the ButterCMS' documentation, the search URL is the following
GET 'https://api.buttercms.com/v2/posts/search/?query=buttercmsapi&page=1&page_size=10&auth_token=your_api_token'
However, using the search function according to docs from the Java SDK calls the following
GET https://api.buttercms.com/v2/posts/?auth_token=token&query=test HTTP/1.1
The /search/ part is missing.