De 'fout' zit bij het Gast-account:
When this Password Protected Sharing is turned off then Guest is
enabled. If the Guest has a password set, then Guest account doesn't
play the guest account role consequently, which can cause Password
Protected Sharing fail to turn off.
To resolve this issue, try following the steps to ensure no password has been set.
1. Right-click Computer and click Manage.
2. Under Local Users and Groups, click Users.
3. Right-click Guest, click Set Password, and then click Proceed.
4. Leave the Password fields blank and click Ok. This will ensure no password has been set.
5. After that, try turning off Password Protected Sharing and see if it works then.