Password not accepted, PW reminder not working

(Fresh Wordpress install with Multisite and Buddypress)

Hi,

we found out that users didn't receive a password on account creation and decided to install the Set Password plugin.

However:

- Users can chose a password on register now, but they are not able to login with that PW.

- If they use the password reminder, they get a "user name or email address does not exist" error. The user account is activated though and has a valid email address which successfully received the activation email initially.

Thank you in advance!

Edit 01: The error on password reminder does not seem to be related to the plugin. I disabled it and the reminder is still not working.
Edit 02: If I (logged in as super admin user) change the password of the user, they are able to log in with the new password.