Apple broke MIDI output in iPadOS 14.2. Because of this error messages can (randomly) occur when transport starts, when you press a Rec button or any other time the audio engine starts.
To work around this issue you can go to Studio --> Devices and select None as MIDI Out Device.
Looks like iPadOS 14.3 will fix it again.
Giel Bremmers
Workaround for error messages in iPadOS 14.2
Re: Workaround for error messages in iPadOS 14.2
The problem is gone in iPadOS 14.3.
Giel Bremmers
Giel Bremmers