Barman backup fails after upgrade to PostgreSQL 10 - Mailing list pgsql-admin

After my upgrade of PostgreSQL 9.6 to 10.3 the backups through barman 
fails. There is not much data change in the database but I still do not 
want to loose the data. I have verified the configuration and all seems 
correct for Barman and PostgreSQL . I have tried to find the answer on 
the internet but I was not able to find it. Both Barman and PostgreSQL 
run on the same server.

PostgreSQL    : 10.3 (Debian 10.3-1.pgdg90+1)
Barman          :  2.3
Debian           :  9.4

Configuration for the database in /etc/barman.d

description = "Productie PostgreSQL Database"
conninfo = host=maat user=barman dbname=postgres
streaming_conninfo = host=maat user=streaming_barman
backup_method = postgres
streaming_archiver = on
slot_name = barman
retention_policy = RECOVERY WINDOW OF 4 WEEKS

When I run 'barman backup all' I get the following output:

ERROR: Backup failed copying files.
DETAILS: data transfer failure on directory '/srv/share/backup/barman/maat/base/20180317T233002/data'
pg_basebackup error:
pg_basebackup: initiating base backup, waiting for checkpoint to complete
pg_basebackup: checkpoint completed
pg_basebackup: write-ahead log start point: 2/B000028 on timeline 1
pg_basebackup: starting background WAL receiver
pg_basebackup: could not connect to server: FATAL:  number of requested standby connections exceeds max_wal_senders
(currently2)
 
pg_basebackup: removing contents of data directory "/srv/share/backup/barman/maat/base/20180317T233002/data"
Starting backup using postgres method for server maat in /srv/share/backup/barman/maat/base/20180317T233002
Backup start at LSN: 2/A1F2CF0 (00000001000000020000000A, 001F2CF0)
Starting backup copy via pg_basebackup for 20180317T233002
maat 20180317T233002 - FAILED

In the barman.log there are the following lines around this message (indicated with ....):

2018-03-17 23:29:02,673 [15604] barman.wal_archiver INFO: No xlog segments found from streaming for maat.
2018-03-17 23:30:02,107 [15646] barman.wal_archiver INFO: Synchronous WAL streaming for barman_receive_wal: False
2018-03-17 23:30:02,326 [15646] barman.server INFO: Ignoring failed check 'backup maximum age' for server 'maat'
2018-03-17 23:30:02,359 [15648] barman.wal_archiver INFO: No xlog segments found from streaming for maat.
2018-03-17 23:30:02,560 [15646] barman.backup INFO: Starting backup using postgres method for server maat in
/srv/share/backup/barman/maat/base/20180317T233002
2018-03-17 23:30:02,662 [15646] barman.backup_executor INFO: Backup start at LSN: 2/A1F2CF0 (00000001000000020000000A,
001F2CF0)
2018-03-17 23:30:02,666 [15646] barman.backup_executor INFO: Starting backup copy via pg_basebackup for
20180317T233002
2018-03-17 23:30:04,069 [15646] barman.backup ERROR: Backup failed copying files.
....
2018-03-17 23:30:04,870 [15646] barman.wal_archiver INFO: No xlog segments found from streaming for maat.
2018-03-17 23:30:05,188 [2390] barman.command_wrappers INFO: maat: pg_receivewal: finished segment at 2/B000000
(timeline1)
 
2018-03-17 23:31:02,041 [15691] barman.wal_archiver INFO: Found 1 xlog segments from streaming for maat. Archive all
segmentsin one run.
 
2018-03-17 23:31:02,042 [15691] barman.wal_archiver INFO: Archiving segment 1 of 1 from streaming:
maat/00000001000000020000000A
2018-03-17 23:32:02,640 [15708] barman.wal_archiver INFO: No xlog segments found from streaming for maat.

The lines with 'INFO: No xlog segments' appear every minute.