Acymail - blank page when registered

I got a really weird problem with SEF Advanced and Acymail.

Acymail is excepted in SEF Advanced as not sef_ext is available as yet  ???
Now this is what happens:

If I use
www.c-stab.net/service/newsletter/user.html

I get to the subscription page as it should be..... EXCEPT if I am logged in.... As soon as the user is logged in to the website, this address results in a blank page.

Any idea what might be going wrong here?  This has not been observed before the installation of SEF Advanced - so I guess it is related - or is it?  Any help is appreciated (as well as an acymail sef_ext )

It's impossible to tell without actually logging to your site and checking. Please send the login info and FTP on mail and I will take a look.

But before you do that make sure you have error reporting on. A blank page can be error page but you can't see any error: www.sakic.net/support/faq/general/#5

Thanks Emir,
you already have the access data.... It is the same as for the other website that you wanted to look at