After upgrading to 2026.2.708, RadAsyncUpload file uploads fail with HTTP 403 Forbidden originating from the new CSRF validation (AsyncUploadHandler.ValidateCsrfToken). The handler returns an HTML error response.

The failure occurs only in our multi-host (load-balanced) environment. In multihost environment as well Issue is inconsistent. The exact same build works correctly on a single-node local/dev machine.

Refer support ticket raise for the Issue to get more detail.

RadAsyncUpload fails with HTTP 403 (ValidateCsrfToken) in multi-node/web-farm environment after upgrade to 2026.2.708 | View Ticket | Your Account