1.copy all /bill01/oracle/archive/BILLING/* files to other location.
2.export ORACLE_SID=<YOURDB>(in windows set ORACLE_SID=<YOURDB>)
rman target /
crosscheck archivelog all;
delete archivelog all;
alter database open;
I guess you already had checked this out, but as I did see nothing about on the replyies or suggestions:
Had you checked if the permissions to write/read at /bill01/oracle/archive/BILLING/ were removed?