Grant request is invalid

I updated the plugin to version 6.0.2 and the problem persists.
I’ve already tried “reconnecting” the account, it failed.
I’ve already tried “disconnecting” and creating a new connection, it failed.
I’m still getting the error: “Grant request is invalid”

In Settings, on the Authentication tab, I only see the “Reconnect” and “Disconnect” buttons. Every time I try one of these options, I go to the authorization page (as in the documentation) and confirm, I go back to the Settings page, on the “Posts” tab and see my accounts connected and enabled. Apparently, everything is ok.

However, when I access “Authentication” again, the tab loads other options below the buttons, but then the whole page disappears and only the two buttons remain visible.
My accounts are still there in “Posts”, all enabled, but the error persists.

I’ve already cleared the caches, I’ve already tried incognito mode and nothing solves it.