So.. currently it seems there is a bug with the TOTP provider, maybe in combination with the Google Authenticator app, idk.
First of all: You need SSH access to your Nextcloud instance.
My backup code wasn't working also.. i havent tested this further, maybe my mistake.
1. Switch to your NC root directory, example:
(2. Run the occ command to show the users providers):
php occ twofactorauth:state USERNAME
3. Disable TOTP for the user:
occ twofactorauth:disable USERNAME totp
4. Disable enforced twofactor:
If you have enabled the option to enforce all users to use two factor, you have to disable this too. This time via FTP, go to your Nextcloud root dir again, open your config/config.php and set 'twofactor_enforced' to false.
Now you should be able to login without two factor.
Hint: Depending on your server setup you probabaly have to point to the right php version to use OCC commands, in my case it's:
/usr/bin/php7.1 occ twofactorauth:disable USERNAME
Shocked emoticon, by: BilliTheCat