Are you sure that it is an error 404?
If that is so, the URL you have been using is probably wrong.
If - instead - a message "could not connect" or "connection timed out" has occured, this is the problem announced in the news on our homepage: There seems to be a peering issue on one of the Internet nodes that relays Internet traffic from you to our server. This affects only few Internet providers, most of our users can log in regularly.
Our hoster has already contacted thos node's IT team, but they have not yet solved the issue. Therefore, our hoster has implemented a temporary workaround so that SoSci Survey should be accessible from everywhere, again, by now.