I faced with that error while I was trying to take rman backup to our new NFS mount point.

The complete error message is:

RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03009: failure of crosscheck command on ORA_DISK_1 channel at 10/04/2019 20:14:34
ORA-19587: error occurred reading 512 bytes at block number 1
ORA-17500: ODM err:KGNFS READ FAIL:Permission denied
ORA-27041: unable to open file
SVR4 Error: 13: Permission denied
Additional information: 7
ORA-17503: ksfdopn:6 Failed to open file /orabackup_nfs/SASKI_dbfull_20191003_8puddo78_1_1.rman
ORA-17500: ODM err:KGNFS READ FAIL:Permission denied

We are hitting this error because of  oranfstab file (which is under $ORACLE_HOME/dbs path) refer to the old NFS point.

After editing this file with new information, an error has been disappeared

-bash-4.4$ more oranfstab
#server: 1xx.xx.xx.xx
#nfs_version: NFSv4
#path: 1xx.xx.xx.xy
#export: /sharedstore/sharedstore mount: /sharedstore
server: 1xx.xx.yy.zz
nfs_version: NFSv4
path: 1xx.xx.yy.zw
export: /orabck mount: /orabackup_nfs