This message can mean different things. Most likely the file is not transported into the right location for any reason. Could be disk space or an issue with the source file. Also a software or configuration issue is possible from what you've provided so far.
If there is sufficient space, provifde debug.log, oracle8.log, full session log and RMAN script used for backup. Why do you use the Oracle integration over the SAP R/3 integration using brbackup?
This is not an ASM setup? If yes, this could be the issue.
Could you please provide the output of ls -la /var/opt/omni/tmp from the client in question. If there are no files located in this folder, run watch -d 'ls -la /var/opt/omni/tmp' during archive log backup time to see if files are created and/or deleted. Have an eye on user names and permissions. Maybe there is something wrong with the user configured.
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