Opened a ticket at host support. Not sure there is something wrong there, I can esily load a file in the root of this domain, seems only /forum folder and its content can't be loaded.
Forum is working now, it was a temporary issue. But I think is important to investigate about this happened, I'm afraid it could happen again. I've read similar issues reported in this forum. Host issue? Platform bug? Server problem? I'm not a pro, that's why I'd like to understand.
Thanks @Brogan and @Mike for the super-fast reply.
If it randomly started and stopped without any involvement from you then it is likely server related.
There is nothing in the core software which would account for that.
Your host would be the best person to speak to in order to understand what happened.