when we first install CB does it sync all users?

Yes.

when I checked under user management most of the users were already there (including the user that I´m using to test). I did the sync anyway but the problem remains unsolved. Since your reply until now I didn´t receive the email (I checked the spam folder).

I don't know what more to suggest. Something is blocking it. It could be a 3rd party Joomla user plugin blocking the store and interrupting the entire process. When you try to use forgot login does it display the "New User Password created and sent to your email address EMAIL_ADDRESS_HERE. Please check your email (and if needed your spambox too)!" message? If not the process was interrupted. Navigate to Extensions > Plugins and unpublish all User plugin types except "User - Joomla!" then retry.

If CB Auto Actions is installed try unpublishing it then retry. If you're acting on profile update triggers this could cause problems as forgot login will trigger those.

If you're seeing that confirmation message the password should've changed and the email process would've fired. Suggests that the email is the only part of the process that is failing. Change the users password to something you can confirm works then use forgot login and see if the known password still works. If not then the password reset fine and confirms it's only the email part of the process that is failing.

No other emails are failing to send? Registration emails work perfectly fine? Seams odd only 1 email would fail. It's possible your mail server is flagged them as spam and blocking it maybe. Try changing the email subject and body using language overrides and see if that helps. The default language strings are as follows.

Key: UE_NEWPASS_SUB
String:
Key: UE_NEWPASS_MSG
String:
Let the default English usage apply (switch your site to English). The email subject and body use sprintf. If the sprintf usage in the string is wrong that's where it could be erroring.

Please PM backend super administrator login credentials and will take a look.