Okay, I think I found a workaround. I'm able to access the website only if I type the complete address:
www.spcnet.tv/forums/. If I don't do it this way, my mobile devices (tablet & phone) return an Appache2/Ubuntu error message.
However, I still can't start new threads on my PC. What I can do is access the mobile version of the site on my tablet and start new threads through that. What's strange is that I can use my PC to reply to existing threads, which I'm doing now typing this (on Firefox). Weird.