I’ve finally had it with the old host with non-working subdomains and sometimes domains – SIEGE is now on a brand new host, which, although more expensive, is also (at least according to my first impressions) much more reliable.
Note that subdomains may still be inaccessiable for the next couple of hours while the DNS cache gets flushed on the nameservers – you can try doing it manually if you want, but it may/may not help depending on the settings of your PC, router and external nameserver (most likely that of your ISP).