Never heard of that one before, but I think I figured it out. How many games do you currently follow?
I checked the behavior of the front page - the page initially loads the header buttons and the news on the right side. Then there is a separate second call to the server (for the record: "ajax_latestleaderboard.php"), to get all the latest runs with regard to your settings: minimum and maximum followed games to display, as well as which games you follow. However, for some really weird reason all those settings are part of this call to the server; so the list of games you follow is part of the query parameters.
It looks like this: fg=19920,691,20710,21099,21459 (The numbers are the games' IDs)
To go back to my question: let's say you follow 1000 games, then all those games' numbers will be sent in the latest-leaderboard request, and the parameter will be at least ~3000 characters long. GET calls to the server are always limited by 2048 characters... and it appears you passed this limit.
If this is indeed the issue, then unfollow some of your games and see if that fixes it. You can also send a bug-report ticket in the support hub about this (feel free to refer to this thread for reference).
The default leaderboard-view of this game is to show IL runs. There is currently a bug on the site that prevents opening IL pages of games or users, if there are too many runs there. It seems this game is one of the unfortunate to get hit.
So while this bug is still unsolved, you can access the full-game leaderboard instead: https://www.speedrun.com/ffxiv/fullgame
After that, you can also access individual levels leaderboards from the side menu (just don't click on "View All").
After looking on all those links you mentioned, I see the problem.
Instead of using the template of "user/[user name]", those links use the bad template of "[user name]".
For example, your pending actions page is https://www.speedrun.com/user/pendingactions instead of being https://www.speedrun.com/user/user/pendingactions
Or in the settings, your notification settings page is https://www.speedrun.com/user/settings/notifications instead of being https://www.speedrun.com/user/user/settings/notifications
Because your username is "user" itself, those links are inherently broken and you can't do anything about it. You can't even change your username if you'd wanted, because that page doesn't work either! Send a bug-report in the support hub, that's the only thing you can do at this point. Hopefully they can help you soon, otherwise you really are in a predicament here...
(Feel free to redirect to this thread as reference)
In my case, the links to those action pages are:
Edit profile (no explicit username): https://speedrun.com/editprofile
Edit theme (similar to what you did): https://www.speedrun.com/edittheme/user/Oreo321 I guess in your case you should use the link: https://www.speedrun.com/edittheme/user/user
Settings (no explicit username): https://www.speedrun.com/settings
You say that your menu buttons redirect you to pages with different templates? What happens if you use the links I wrote above?
@Mebiusdore What you describe is not a speedrun, it's a "challenge run". Players can get better and better at the game, and eventually challenge themselves to get perfect combos/score/whatever. But essentially, they still "complete" the game in the same time it can take for another player who don't get perfect score.
Then go directly to this page: https://www.speedrun.com/settings It's really not clear what you are doing and what goes wrong.