DevGateway Fails with XML Deserialization Error: Cannot deserialize XML:

Hello Anonymous,
Thank you for reaching out to the Microsoft fabric community forum. The error that your see it might be cause of XML Structure or Namespace Issue, Configuration Mismatch or Unescaped Characters.

Please consider the below troubleshooting Steps:

• Open WorkloadManifest.xml in a text editor like VS Code to make sure it starts exactly with. There shouldn’t be any extra spaces or hidden characters before this line.

• Compare your WorkloadManifest.xml with the sample provided in the Workload Development Kit. Ensure all required elements (e.g: <AppId>, <RedirectUri>) are present and correctly formatted and Use an XML validator to check against the WorkloadDefinition schema.

• Go back to the untouched sample WorkloadManifest.xml from the kit and test the registration process with it. If it works, compare it side-by-side with your modified version to spot any differences causing the issue.
• Check the DevGateway logs for more details on the BadRequest error. Look for hints about what might be wrong, like an invalid AppId or other fields that aren’t lining up.
• If you’ve added custom names or descriptions in WorkloadManifest.xml, make sure they don’t include characters

• Check you’re using the latest version of Microsoft.Fabric.Workload.DevGateway.exe from Microsoft’s Download Center.

If this information is helpful, please “Accept as solution” and give a "kudos" to assist other community members in resolving similar issues more efficiently.
Thank you.

Hi Anonymous - happy to see you are taking the WDK for a spin 🙂 

Your message suggests that you only made minor modification and haven't yet registered an Entra app and some additional must-do updates to the sample manifest package: Quick start - Run a sample workload - Microsoft Fabric | Microsoft Learn
After making the changes don't forget to run the DevGateway with reference to the re-packaged (step 11) manifest package (.nupkg): https://learn.microsoft.com/en-us/fabric/workload-development-kit/quickstart-sample#step-12-run-the-devgateway-file. 
Please update us if this helped or any additional errors you encountered.