I don’t have memberpress, but the same (or similar) issue is happening to me.
When the user logs in, instead of the page to enter the 2FA code, it takes them to a page that just shows the [uo_login_ui] shortcode.
This seems to have started in the last week as well, so I am assuming an issue with 3.8
Bluehost. This is bringing down our sites multiple times per day – is there a workaround?