@RieDaniel-6883 - 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.
Ask: Issues Fetching Access Token in Azure DevOps Release Pipeline for Azure Synapse Workspace Deployment
Solution: The Synapse workspace deployment task does not support workload identity. Therefore, the service connection must use a secret for the service principal authentication.
This documentation gave us the hint: https://learn.microsoft.com/en-us/azure/devops/pipelines/release/troubleshoot-workload-identity?view=azure-devops
As workload identity federation is considered best practice, it would be great if Microsoft could update the Synapse workspace deployment task to support it.
If I missed anything please let me know and I'd be happy to add it to my answer, or feel free to comment below with any additional information.
If you have any other questions, please let me know. Thank you again for your time and patience throughout this issue.
...............................................
Please don’t forget to Accept Answer and Yes for "was this answer helpful" wherever the information provided helps you, this can be beneficial to other community members.