Another nice one:
Since I'm using the old version of wp https and the latest version of contact form 7, everything worked wonderful - except in combination with Contact Form 7 - Dynamic Text Extension.
In my case, dynamic content is being pasted and finally submitted by the user via the form. Using the page with https means, that the "@" is getting lost from a e-mail address.
Going the same way without SSL secured website, it works like a charm.
I know development takes a lot of time and people do it mostly for free here, but it's really a mess if you have to double check every little peace to make sure your website is not only working half way after running a update.