I applied an "Alternate CSS URL" to all sites and subsites inside 'Site Master Page Settings' for my site:
This has caused some serious issues in the nintex forms application, with the #contentBox being the most affected (I think). The main issues are overlarge left and top margins. The biggest issue is the top margin, where it impacts on the ability to use all controls.
Any help would be awesome,
NB: De-activating then Activating Nintex Forms from the site collection features does not resolve the issue.