I tried opening my project in Unity 2021.2 (a11 to be precise) and now all custom timeline tracks show "Track cannot be loaded" warning. They have been working fine since 2019.2. I re-imported the timeline asset and I got this warning. Re-importing the related scripts gave similar warnings. Closing Unity, deleting Library, and then starting Unity again makes the tracks work for a while. I can enter the play mode a few times before the tracks suddenly cannot be loaded again. Only deleting Library folder seems to fix it. Is there some upgrade step that needs to be done? Or is this simply a bug?