Issue connect() failed (111: Connection refused) while connecting to upstream

[Azurel]

I get very often this errors in proxy_error.log:

[error] 19343#0: *2462301 connect() failed (111: Connection refused) while connecting to upstream, client: xxxx
[error] 19875#0: *2462746 recv() failed (104: Connection reset by peer) while reading response header from upstream, client: xxxx


CentOS Linux 7.3.1611 (Core)‬
Product Plesk Onyx Version 17.0.17 Update #18
Server version: Apache/2.4.6 (CentOS) built: Nov 14 2016 18:04:44

What can I do to fix this?