The ACP.BoltwoodFile really goes out of its way to detect and prevent file locking conflicts. Look in the WSC starting at line 545. The so-called Boltwood standard actually includes locking requirements. There should be a file My Documents\ACP Astronomy\Logs\BoltwoodFileTrace.log. Hopefully you still have the one from when the problem happened. See if there is some evidence there that can help with this. If it has been overwritten, let's wait till it happens again and then look.