HP34401 Read Meas.vi has a TimeOut input. If this TimeOut is not long enough, the VI will consider that HP34401 took too much time to answer to the query. Try increasing the timeout. I think 500ms is about to be the minimum for this input (according to the multimeter I've got - HP34401A). The -401 Error occurs in the multimeter when you send a query without emptying its buffer first. You probably have this problem when you rerun the VI after having error -1073807339 with LabView. This is just because the multimeter took too much time to answer so that you didn't read its last answer.
Hope this helps ! Julien
