What what it's worth, I think this may be a "cookie" issue.
I refreshed my browser this morning after leaving it last night and received the "unavailable" error.
So, then I launched a new "incognito" browser and navigated to Disboards. The site came up perfectly fine. I went back to my main browser and refreshed to an "unavailable" page.
I then cleared my cookies and cache on the main browser. The site was available again.
I have a feeling like there may be a date/time stamp may be being written to a cookie and it's not being updated again after some time causing the error.
If it was just me, I would say that it's my browser/computer causing this cookie corruption.
Since it's more wide spread, it may be the site code. Were there any updates recently?