Different ISPs/IP addresses, different OS'es and different devices did nothing. No change for me.

So I created a new ffnet account.

I ended up having to create a new email address also first, since I couldn't create another ffnet account with the same email address. Huge hassle, and at first they wouldn't let me post anything on the new account, since it wasn't verified and I wasn't getting the emails to do so.

After changing the email address AGAIN, I got the verification mail for the new ffnet account, and I can post stories in the new account!

So my problem is sort of solved - but my previous ffnet account is still as it was, and nothing new can be posted from that.

I never got a reply from ffnet support, email or twitter. I guess I shouldn't have expected one.