We already saw rhat error and everytime is was due to another plugin. Try to disable security plugins for example, but it could be any plugin. Let me know if you find which plugin is creaking the Subscription so I can contact the author.
I found the culprit: WP-SpamShield latest Version 188.8.131.52 | Von Scott Allen
If I disable it, subscription works. If it is enabled, I get an error message! If they can fix the problem, it would be great if someone can let me know, I’d love to activate their spam protection again.