The information you provide above shows the following error in our logs: Authentication failure: IDX10501: Signature validation failed. Unable to match key: kid: 'System.String'.
Within our iOS distribution you will find our Swift sample app, dynamic/SinchRTC/samples/VideoCallKitSwift: https://download.sinch.com/ios/5.12.5/SinchRTC-iOS-5.12.5%2b6f51d567.tar.bz2 There you will find an example for how to create the JWT with Swift and you can also run the sample to verify you credentials.
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