I have been using WordPress for a few years now with only a handful of plugins. It seems after the latest update for Jetpack that comments no longer worked and gave the below error:
Error: empty answer. 
I checked my plugins and the only one that I thought could be causing the issue was Anti-spam. This plugin was deactivated and the error resolved.
I think the latest update for Jetpack has issues with the Anti-spam plugin.
This worked for me but please leave a comment on what resolved this issue for you or if you found this post helpful
By Paul Wood