Hi, we're sorry to hear about this issue. However it seems to be a Microsoft security issue and not necessarily a bug in our app, though we haven't counted it out. Thankfully this problem doesn't come up often, but we're still not 100% sure why. We have been able to help some users with this issue, so hopefully we can help find a solution.
Basically, it's saying you have access to the machines like you're saying, even the ADMIN$ share, but it's not allowing remote service changes (try the SC command in a DOS Prompt), which is also required for scanning. Usually if one has access to ADMIN$, they can also start/stop remote services, just not in these rare cases.
I'm assuming this is on a domain. Are you running it from a Domain Admin account, or are you needing to use credentials in NEWT in order to scan, on PCs where it's working?