@örg Mayer Azure Trainings I'm glad that you were able to resolve your issue and thank you for posting your solution so that others experiencing the same thing can easily reference this! Since the Microsoft Q&A community has a policy that "The question author cannot accept their own answer. They can only accept answers by others ", I'll repost your solution in case you'd like to "Accept " the answer.
Issue: Unable to Mount Azure Files share on non domain joined VM
Error : 86. Connectivity to the domain controllers is available,
net use Z: <YourStorageAccountName>.file.core.windows.net<FileShareName> /user:username@domainFQDN
Solution: FQDN of the storage account from the non domain joined pc helped to resolve the issue
The on-premises DNS server resolves IP addresses. However, Azure DNS resolves the Azure file share Fully Qualified Domain Name (FQDN). All DNS queries to Azure DNS originate from the virtual network. There's a DNS proxy inside the virtual network to route these queries to Azure DNS. For more information, see On-premises workloads using a DNS forwarder.
Based on the error message there are few threads which can resolve your issue: System error 86 has occurred. The specified network password is not correct.
Please let us know if you have any further queries. I’m happy to assist you further.
Please do not forget to "Accept the answer” and “up-vote” wherever the information provided helps you, this can be beneficial to other community members.