Authenticated users can't unsubscribe.
I have them subscribe using the "Simple Opt In" module. When they want to unsubscribe, they are sent to the "Opt In Email" module. However, this page shows a blank page. It does show up when the user is not logged in, but, when they click the link in the verification email to unsubscribe, again it shows a blank page. And they are not unsubscribed....
Edit: it appears that unsubscribing isn't working at all anymore. Not for authenticated users, and not for anonymous users.
Edit2: I can now also not send any test email. I also tried not checking an optin list and just try and email to 1 address in the 'additional emails' field, this also doesn't work. Also, if I don't send immediately, but on a schedule, nothing happens. Everything seems to be broken! 
It worked fine before. I have no idea what has happened. I haven't made any changes to it. All I can think of is that I installed Ifinity Friendly Url provider a couple of days ago. This gave me lots of problems with several modules, so I totally removed that again. Maybe that somehow messed up Opt In Email? But that seems strange to me, since I have totally removed it. Also put back a backup of web.config.