Hi, it sounds like your blog has a block at firewall level when connecting to http://www.thenewsletterplugin.com. Ask the provider to check, if possible. Let me know!
there is no Block. I deactivated Wordfence Security. Nothing. Provider also nothing.
I had 13 Updates – Newsletter updated fine. Some plugins of Newsletter, too.
Only 7 are still pending and not able to update.