Error #6 is Invalid Argument, a fairly general error that means we got some bad juju in a request. One common way to generate it is to attempt to display status on an out-of-range array reference (the V index points beyond the block). Another possibility would be that the project in the PLC doesn't match the project that DmD has loaded (moved the cable with an active session) then attempted to read status of a heap item or block that didn't exist. Since you had an array reference in rung 10, and the problem only shows up when you show status on rung 10, it just followed that it might be a bad array reference. Depending on the state of ST18, we either treat out-of-range array refs as an error or a warning.