i beleive your Your control file has the references to
the out-of-date archive log file. You need to delete
the references from the control file. Perform the
following: ( assuming u are not using rec.catalog)

1. Type "rman target <user_name>/<password>@<connect_string> nocatalog" 
2. Type the following command: "list copy of database archivelog all;" 
This command lists all of the archive logs 
3. You need to remove any archive logs that exist in the resulting list that do not exist on the disk drive. You will need the key value (first column in the list). Type the following to delete an archive log: "change archivelog <key> delete;" 
You can also use a comma separated list if you have several archivelogs to delete. 
4. Rerun the command in step 2 to verify that the archivelog has indeed been deleted. 
5. Exit RMAN and retry the backup.