Previously, this topic was addressed here:
The problem is still ongoing, but at least it's been identified for what it is: Server hacking. Someone is dropping php files on the server and using them to DOS someone else - which uses up all bandwidth and makes it look like the server has gone down.
I'm currently working with server management to try and find the source of the problem.
Meanwhile, someone suggested that file-upload scripts are often targets, so I have temporarily disabled files.simutrans.com to see if that might be the vector. If so, I will address it later. If not, I will re-enable it later.