Login fails / update fails

After updating to version 6.15 nobody is able to login anymore, even user 1.

To login: You can type in username and password, but the login fails without error message and returns to the login screen. no further reaction.
Update: After changing setting.php the update screen to select the versions get up. After pressing the Update button the update screen gets up again. no action, no reaction.

The cause (maybe):
After running the update process the first time I got an error (memory allocation failed).

Now, roughly 1.500 users are locked out, including myself.
Quick help is welcome!