Thanks for your answer, Kryten, but I think that such a basic functionality needs to work. Today I could log in. Apparently, they had something broken in their servers.

Apparently, logging in from the CubeIDE works right now. But I also experience the problem that I cannot run an example project because it is looking for libncurses5 whereas this Ubuntu only has the libncurses6. 

I have the same problem under Ubuntu 24.04. I have direct Internet connectivity so I don't need a proxy. It just times out while trying to open the log-in page. I installed the application using the generic Linux installer. (The Debian one complained...