You receive the error message ‘Only one instance of the Array Configuration Utility can be running’ when opening the ACU utility on a HP Server.
This occurs when no other instance of the ACU utility is actually running.
It would seem that this effects, rather infuriatingly, Windows 2003 and 2008 servers with Internet Explorer 8 installed and ACU Version 8.25 (or earlier) for Windows 2003 and ACU Version 8.20 for Windows 2008.
To resolve this the version of the ACU utility needs upgrading to version 8.28 for Windows 2003 and 8.25 for Windows 2008.
Alternatively, you could uninstall Internet Explorer 8 and roll back to an earlier release.
HP have released an advisory here http://h20000.www2.hp.com/bizsupport/TechSupport/Document.jsp?objectID=c01763057