Thanks for all your replays. As I always buy a yearly membership (but not every 12), it's likely that this "feature" is in place since a year. I personally relay hate that!
I tried the gift membership but it does not work on the site and as I don't live in the u.s., the real gift cards are no option for me.
So I went on and buy one whit auto renewal. I tried to cancel it right after following the description here in the forum. But there was no option to do so. Only now, after several days, the option shows up at the exactly same spot where I checked before.
Also the answer from the suport was not helpful and they don't replay to additional mails.
I'm really disappointed by Groundspeak.
Here is how I did cancel the auto renewal:
Open https://payments.geocaching.com/membership/edit
Cklick on "Stop Recurring Membership."
Wait a few minutes as the site is crappy slow.