I'm using ES 5.0.1 with X-Pack with only Monitoring activated on a basic license. In Kibana, if I go to Monitoring -> My cluster -> Elasticsearch -> Indexes, I get the error in the title. I checked (briefly) with tcpdump, and I appear to be indeed getting a JSON-formatted response from Elasticsearch when I click the page. The response is huge though, so I'm not sure if maybe there is a formatting error in there somewhere.
This used to work properly yesterday, with no changes in the meantime except indexing more data. I have 260 indexes currently. Has anybody else hit this problem? Any help is greatly appreciated.