You are backing up to disk, so I don;t know exactly why you need to do a "backup validation" before you do a backup.
You'll need to post the script you are using to do the validation and the backup.
If the rman script is runing successfully completed second time, it means there is no issue with Script and you can ignore it.
If you want to check the database before backup, you can use the RMAN> database validate;
but when a backup RMAN, it will check the database files during the backup.
Backup validate is to check the status of your backup thats mean after backup and when you want to restore.
while your backup is done , means ok but you can display your scripts .
RMAN-03009: failure of backup command on ORA_DISK_4 channel at 12/07/2018 07:21:20
ORA-03113: end-of-file on communication channel
channel ORA_DISK_4 disabled, job failed on it will be run on another channel
Your backup will be OK because RMAN resumes on another channel. You might however have unnecessary I/O, time and space usage.
It's not possible to tell the reason from the current output. What are these channels? Network, local disk, tape? It could be that the storage path did not respond was disconnected. Keep in mind that parallel processing is not necessarily an advantage if you do not have the necessary hardware, in which case you may "oversell" your system and cause processing delays and overhead.