Hi Bettina Höß,
Reviewing through you description,
"We have received a request from 184.108.40.206 to reset your password. If you asked to reset your password, please follow the instructions below. If you do not want to reset your password, disregard this message. Your user name and password can't be managed inside Jive because they're controlled by an external identity provider, such as LDAP or Active Directory. Please contact your administrator for help."
it appears you are using an external identity management system in your instance (LDAP/SSO). As such, your Jive instance will not be able to reset the password for an account as these will be managed in that external system. As such, I would recommend reaching out to the team who manages that system, as they should have an option to update a user's credentials.
Or, if you are not using an external identity management system in your instance, I would suggest submitting a case in your customer group ( MyJive Secret Support Group Administration FAQ ) as a next step, so that a member of our support team can perform further investigations specific to your instance.