Login wordpress only works the second time

Hello,

My webshop, and many other sites, was hacked 2 months ago. After a few weeks I discovered this thread: https://wordpress.org/support/topic/tmp-theme_temp_setup-what-is-this and followed this advice that was given. I regained access to my site and I thought everything was back to normal.

However, when I try to login on wordpress I have to fill in my username and password 2 times. The first try I’m getting this error: ‘Invalid username or email’ while I’m 100% sure that the username and password are correct.
This only happens the first time. When I login, logout and retry to login I’m not getting this error.

As I lost my site 3 weeks due to this hack, I’m scared the hack has left some code on my site that is causing this problem. I checked the functions.php and wp-login.php files and they don’t contain any code that was mentioned in the thread.

Does someone how to fix this problem or what php-file is causing this problem?

Thanks in advance!