Upgrading may not be possible at present as the master is a 24/7 operation, but if anyone knows whether master and slave can be different minor releases (9.5.4 vs 9.5.6) would be helpful
What is your archive_command and full_page_writes set to? Also, what is the value of checkpoint_segments and checkpoint_timeout?
checkpoint_segments - Not found in params, could this be
wal_keep_segments = 48
checkpoint_timeout = 5min
archive_command = disabled
full_page_writes = on
Try increasing wal_sender_timeout before running pg_basebackup.
I have just increased the value from 1min to 5min
I'm looking also at other backup options such as barman