Hello,
The issue occurs because of the Full Name attribute. Active Directory requires that objects located directly under a particular container to have a unique name. For users, their Full Name serves as the name of their AD account. Active Directory does not distinguish umlauts in object names, which means that for AD, Rita Öxle is the same as Rita Oxle. In other words, you won't be able to create users with such names in the same AD container, either via Adaxes or using AD native tools, such as ADUC, for example.
To work around this issue, we recommend using a script that renames a new user if their Full Name is not unique in the target container. You can also find it in our tutorials. See Validate/Modify User Input Using a Script, Step 5, Example 3. The script does not distinguish umlauts in full names, the same as AD.