I ended up rescheduling my exam instead.
It seems that it's still not fixed today.
I opened a ticket with Red Hat Training Support. They said that they're experiencing a global outage and that the concerned team is working on it. No ETA though.
In general, there were a lot of global outages during the last few weeks, in regards of lab command (start/finish), and in regards of the enviroment itself (unable to start the lab hosts, or to build them).
I also opened an incident, I hope I'll get a solution until tomorrow evening, so I can take my exam on Thursday.