Some specific storages don't support flock() file locking with their NFS implementation so the workaround is to disable flock() on the mount with nolock parameter. If it doesn't help, then please change nfsvers=4 to nfsvers=3, it should help. This issue is particularly known for DataDomain storage.
Use the following command to mount NFS: mount -t nfs -o nolock,hard,intr,nfsvers=3,tcp,bg xxx.xxx.xxx.xxx:/aa/bb /dd/storage/backup
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