I'm not sure that I can help. Your site is still working here in the UK (Nabble's servers are in Texas, I believe.) and responding extremely fast.
From what you describe, it does begin to sound to me that the site may be blocked somewhere. However, I don't think it is a Nabble issue and you will need to investigate causes elsewhere, perhaps start at the ISPs of the complaining users?
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.