The problem is the last / after tasks in your URL. If you are putting slash at the end, it will resolve into the URL that expects task id. Which you did not provide in the URL. So you are getting the error.
If you wanted to make a PUT request without the task id, you have to call /api/private/tasks not /api/private/tasks/.
What would help here to diagnose the problem maybe a little bit more easily would be to include the name of the resolved API from the model and also its modeled URL. Something like:
2025-11-07 11:18:23 2025-11-07 11:18:23,263 [33]
ERROR Origam.Server.Pages.UserApiProcessor -
Error occured (System.Exception) for request:
2025-11-07 11:18:23 localhost//api/public/tasks/:
2025-11-07 12:26:33 Error occurred while processing API page
Name: /api/private/tasks/{taskId}
Url: /api/private/tasks/{taskId}
Exception: Error occured when setting parameter: @Task_parId to value:
2025-11-07 12:26:33 —> System.FormatException: Unrecognized Guid format.