Barman backup fails after upgrade to PostgreSQL 10

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 (currently 2)
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 (timeline 1)
2018-03-17 23:31:02,041 [15691] barman.wal_archiver INFO: Found 1 xlog segments from streaming for maat. Archive all segments in 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.