This is on a brand new Flarum install?
I see you had issues with a Flarum install already and got it working. Can you do a few things for me?
If you have Cloudflare, refresh its cache.
Run php flarum cache:clear in your flarum root directory
Hard refresh your browser (CTRL+F5)
Turn on debug (in config.php, change debug's value to true) and try the login again. If it comes up with an error, click DEBUG and paste the error you see in the popup box here.
If these don't resolve the problem, please run php flarum info in your flarum root directory and paste the output here. You can use ``` to enclose the output for easier reading.