It is not in my habit to write negative reviews, certainly not for free extensions but I think an advance warning is in order here. Do NOT install Ninjaboard when you have CD Captcha installed. It made it impossible for me to login to my backend! On their forum I read it is a known conflict... (also provides an answer for people like me who nearly had a panic attack after being denied access to their own site admin ;-) Before installing an extension I usually read the JED reviews, not the product forums... So I hope you will allow this friendly warning to other users. Having said that, I think Ninjaboard looks great and probably works well if you don't have CD Captcha installed. Would be great if the issue can be fixed though, then I will definitely give it another try. I love the clean look and CB Integration (although I didn't get to test it yet)!
I am sorry that they didn't work together, but unfortunately this is not something wrong with Ninjaboard but with the core design plugin.
1. There are quite a few extensions that when installed alongside the plugin your site dies completely.
2. No other plugin kils your site when installed with Ninjaboard.
This is because of how they try to find out what page they are on, using the JApplication object. However this shouldn't be used in an on after route plugin because (as the errors suggest) anything that sets data to the page header will cause an error.
If they instead replaced line 116 of the plugin :
$appname = $application->getName();
$appname = JRequest::getVar('option');
Then none of us would have this problem.