Error starting backup

A customer of mine is running 8.3.8. We are attempting to
take a backup, but whenever we issue pg_start_backup() it
always says that we need to first run pg_stop_backup() - Even
when I run the stop backup command right before trying to
start the backup. I have restarted PostgreSQL altogether,
but the results are the same. Am I missing something?

Does pg_stop_backup() return without error?
What if you call it twice in a row?

Is there a file "backup_label" in your cluster directory?
If yes, what are its ownership and permissions?

Yours,
Laurenz Albe