If you've opened the PST file in Outlook and Outlook is still running then the file will be permanently open and therefore unavailable for being backed up properly. Don't know why the AOFO option didn't do the trick for you, but I'd suggest the easiest thing if possible is to make sure that the PST isn't being used when the backup runs.
Is is available for everyone to access, or is it just accessed from the server (or your machine perhaps) for purposes of archiving? If it's the latter, especially if it's running on the server, then perhaps the easiest thing would be to ensure that Outlook has been shutdown each night before the backup runs. Perhaps get a pre-run script setup to kill the outlook.exe process if it's running, which will then kill outlook and therefore sever the link to the PST file, and with it the lock that is causing the problems.