Changing the admin password2 users found helpful
without the solution! I'll have to reinstall C5!
it is a very serious bug!!
I found it here:
This is how:
Take the salt-value from /config/site.php and run this PHP-code:
input the result DIRECTLY into the database (with PHPmyAdmin or Sequel Pro etc.). It's already encoded, don't md5 it again...
All you need is FTP. No coding necessary. :D
Hope this helps others in the future.