I believe that the error you are seeing is due to a specific VI in your system. It may be a LabVIEW 6.1 bug, or it may be a corrupted VI.
I tried locking a generic 6.1 VI and it worked just fine. To solve the problem you need to narrow down which VI causes the error. You might try distributing a sequence that contains a subset of steps by cutting and pasting from your sequence to an empty sequence. Once you have narrowed it down to a particular top-level VI you can attach it for us to do some diagnostics.