Lumberjack input: the pipeline is blocked, temporary refusing new connection.

We are seeing this warning very often. Any idea how to fix that?

``` bash
{:timestamp=>"2015-11-05T11:07:30.187000+0000", :message=>"Lumberjack input: the pipeline is blocked, temporary refusing new connection.", :level=>:warn}
{:timestamp=>"2015-11-05T11:07:30.688000+0000", :message=>"Lumberjack input: the pipeline is blocked, temporary refusing new connection.", :level=>:warn}
{:timestamp=>"2015-11-05T11:07:31.189000+0000", :message=>"Lumberjack input: the pipeline is blocked, temporary refusing new connection.", :level=>:warn}
{:timestamp=>"2015-11-05T11:07:31.689000+0000", :message=>"Lumberjack input: the pipeline is blocked, temporary refusing new connection.", :level=>:warn}
{:timestamp=>"2015-11-05T11:07:32.190000+0000", :message=>"Lumberjack input: the pipeline is blocked, temporary refusing new connection.", :level=>:warn}
{:timestamp=>"2015-11-05T11:07:32.691000+0000", :message=>"Lumberjack input: the pipeline is blocked, temporary refusing new connection.", :level=>:warn}
{:timestamp=>"2015-11-05T11:07:33.191000+0000", :message=>"Lumberjack input: the pipeline is blocked, temporary refusing new connection.", :level=>:warn}
{:timestamp=>"2015-11-05T11:07:33.692000+0000", :message=>"Lumberjack input: the pipeline is blocked, temporary refusing new connection.", :level=>:warn}
{:timestamp=>"2015-11-05T11:07:34.193000+0000", :message=>"Lumberjack input: the pipeline is blocked, temporary refusing new connection.", :level=>:warn}
{:timestamp=>"2015-11-05T11:07:41.296000+0000", :message=>"CircuitBreaker::Close", :name=>"Lumberjack input", :level=>:warn}
```
- ubuntu 12.04
- logstash version `1.5.5`
- java `1.7.0_85`