buildbot failure in on flink-docs-release-1.5

[buildbot]

The Buildbot has detected a new failure on builder flink-docs-release-1.5 while 
building . Full details are available at:
    https://ci.apache.org/builders/flink-docs-release-1.5/builds/308

Buildbot URL: https://ci.apache.org/

Buildslave for this Build: bb_slave2_ubuntu

Build Reason: The Nightly scheduler named 'flink-nightly-docs-release-1.5' 
triggered this build
Build Source Stamp: [branch release-1.5] HEAD
Blamelist: 

BUILD FAILED: failed Build docs

Sincerely,
 -The Buildbot