So, I obviously managed to do it now (on my phone), but I almost wrote a message to the mailing list because I was pretty desperate.
In no configuration could I manage to create an account on my Mac (Orion, Firefox, Safari, VPN on, VPN off, Private window), always getting the following error message:
![Red bar saying "Your info has been marked as spam" above the sign-up form](https://i.ibb.co/m5wJsqp/Screen-Shot-2021-10-29-at-18-40-15.png)
Are Mac users' contributions usually that bad? :-)