I don't know how.
How many times have you tried it and how long have you waited?
I get that every once and awhile just reading the forums and trying to switch forum, it usually works the next time, sometimes it occurs a few times in a row but never for very long.
It's probably, as the popup suggests, just a random error, though if it keeps happening maybe that particular file is broken, try reporting it
I'm getting the same error when I use your link, I tried a couple of times.
Searching for scare-mod doesn't find anything relevant.
Where did you find the link?
see https://www.minecraftforum.net/forums/forums/forum-discussion-info/2944101-regarding-disappearing-threads-posts-and-users for the reason why the threads gone