Thank you for your quick response!
I use
http://ulm.wolfshire.co.uk/ - I have an account there, it did work last time I did it, so I´m confused right now.
It seems now, that I don´t stay logged in - after I log in, everytime I link to another sub-page of the website, I have to log in again.
I´ve checked my cookie-settings, they seem to be okay (tried it with Firefox AND with IE 6.0, both with the same result)
It´s not so urgent, I just wanted to register another 5 prefixes, so I have some more as a reserve; I don´t really need any in the next few days (I still have 2 prefixes without an age).
I know about the alcugs-list, I´ve edited it myself already to post my ages. I just thought, ULM is safer because it is automatic - I fear that people use prefixes which are not posted in the alcugs Wiki... so I never can be sure, a prefix I choose isn´t already in use. I like to be on the safe side, obviously