I'm experiencing the same issue (ELK 7.8):
{"type":"error","@timestamp":"2020-06-23T14:29:01Z","tags":[],"pid":21238,"level":"error","error":{"message":"Internal Server Error","name":"Error","stack":"Error: Internal Server Error at HapiResponseAdapter.toError (/home/ubuntu/kibana/src/core/server/http/router/response_adapter.js:130:19)
at HapiResponseAdapter.toHapiResponse (/home/ubuntu/kibana/src/core/server/http/router/response_adapter.js:84:19)
at HapiResponseAdapter.handle (/home/ubuntu/kibana/src/core/server/http/router/response_adapter.js:79:17)
at Router.handle (/home/ubuntu/kibana/src/core/server/http/router/router.js:162:34)
at process._tickCallback (internal/process/next_tick.js:68:7)"},"url":{"protocol":null,"slashes":null,"auth":null,"host":null,"port":null,"hostname":null,"hash":null,"search":null,"query":{},"pathname":"/api/metrics/snapshot","path":"/api/metrics/snapshot","href":"/api/metrics/snapshot"},"message":"Internal Server Error"}