First, I have tried 6.20 and experience the same results.
I select "Update All headers", when NBP gets to a few major groups (a.b.teevee, a.b.multimedia for example) it lists headers to DL, but the download count never increases from zero. I let it run all night and it just "churned." Every time I close the program I need to wait about five minutes before NBP exits from task manager. Each time I open to "Update All Headers" these same groups get hung up, but the headers to download grows- telling me no updating of headers has occurred.
Is there any solution beyond re-DLing all headers from these troubled groups?
PS I ran a chkdsk on the data drive. There were no problems.