Our forum is back up again now.
We are sorry for this and, the reason was shockingly mundane: all technicians at our ISP were in vacation last week, so we couldn't contact anyone to report the downtime! This, regardless if their are supposed to offer technical support 24/7 365 days per year.
Note that, while the forum is running normally, it might take some time (up to 24/48 hours) for your ISP to catch up the DNS change from the backup server to the regular one, but it will eventually appear soon. It should be faster if you use Google DNS ( 8.8.8.8 ) instead of the one provided by default by your ISP.