buildbot failure in on syncope-2_1_X-docs

[buildbot]

The Buildbot has detected a new failure on builder syncope-2_1_X-docs while 
building . Full details are available at:
    https://ci.apache.org/builders/syncope-2_1_X-docs/builds/283

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

Buildslave for this Build: bb_slave3_ubuntu

Build Reason: The Nightly scheduler named 'syncope21DocsNightly' triggered this 
build
Build Source Stamp: [branch 2_1_X] HEAD
Blamelist: 

BUILD FAILED: failed

Sincerely,
 -The Buildbot