Here's what I'm doing to get to what I'm seeing:

1) Log out.
2) Click "Account" from the option bar at the top of the page.
3) Click "Login Using Your Maniaplanet Account"
4) I now see a banner across the top of the page that says "Your ManiaExchange account has not been linked to your Ubisoft Connect account yet. Click here to link them to upload replays."
5) When I click as instructed, I get taken to: https://accounts.mania-exchange.com/user/auth_uplay
6) That page informs me that "An error ocurred!" [sic] and "Sorry, you do not have permission to view this page..."

The error message

... says:
"Sorry, you do not have permission to view this page...

is a little remnant of old code and just says that you need to log in (you are now on a different page). So when you see this, you simply need to click on the top right to log in.

Ozon says:
The error message

... says:
"Sorry, you do not have permission to view this page...

is a little remnant of old code and just says that you need to log in (you are now on a different page). So when you see this, you simply need to click on the top right to log in.

Ah, I wasn't even looking up and to the right--I was just assuming it was a bug.
So I was already logged into the site, but I needed to log into a second, different site/domain in order to make things work on the first site?
Okay then.

Seems to be working for now. Thanks for the help.

Edit: I just noticed that after posting this comment, I was taken to:
https://trackmania.exchange/threads/225?page=-1
...instead of my post. Seems odd? I'll shut up now.

Oh yes, that's a real bug haha.

Thank you for the report pjw, it shines light on usability issues on the site. The "permission denied" error message should not appear normally, so what I explained is just a workaround.

Ozon says:
Thank you for the report pjw

...and thank you for the hard work on the site--it's way better than it would be without you.