Hello,
The actual subscription process is handled by JetPack/WordPress.com service – this plugin merely ties into that service. Have you tried using that email in JetPack’s subscription widget? I’d be curious if you see the same error. I would try a few things:
1) Add JetPack subscription widget to your page and try using that to see if you get the same error. If you do, contact JetPack support as it is a JetPack issue.
2) Logout of the dashboard, clear your cache, open your website and try subscribing that way.
3) Use a different email to test with.
Would be curious to hear if you find a resolution. I’m not able to reproduce this defect on my end, but let me know if I can be of any more help.