Hello @fritz.weideneder,
My name is Henry and I am happy to assist with your inquiry, and appreciate you contacting LastPass Support!
I am sorry about the issues you are having when trying to change the Master Password. Which field is grey? Can you please describe the steps you are following? You should be able to change the Master Password from the account settings.
Please let me know.
Thank you,
Henry | Customer Care
If you are referring to the button to complete the change, please make sure that all the requirements have been met and the strength meter is full.
Please let me know if this worked or if further assistance is required.