user.module: !password not generated

Hello,

I just migrated from 4.6 to 5.1. Unfortunately the password request does not work any more. Therefore users who forgot their password cannot login. The sent email says:

Username: Alexander
Password: !password

The password simply does not get generated. (The old password is still valid.)

My site is restricted to authenticated users, therefore it is quite essential to me.
Can anybody help?

Thank you.

Alexander