Cbbackupmgr error logs

While testing cbbackupmgr and creating the initial backup yields log entries in the backup.log file of the next kind:
2017-06-19T16:43:30.326+02:00 ERROR: db handle is being used by another thread -- couchbase.dcpClient.Mutation() at dcp.go:238
These occur every 3~6 seconds.

The command syntax used to do the backup is:
/opt/couchbase/bin/cbbackupmgr backup -c -u Administrator -p admin123 -a /var/mf/cb-backup/cb-backup -r cluster -t 1 --no-progress-bar
No other application was running interfacing with Couchbase.

What do these errors mean?

[Environment: 2-node Cb EE 4.6.1 on CentOS 6.8]

This is a false positive error message which you can ignore. The underlying storage engine is reporting an error, but we have confirmed through extensive testing that even when this error is reported that all data is properly written to disk. In our upcoming 4.6.3 and 5.0 release we have made changes to that this false positive no longer shows up.