Thanks Pokoy. Yes, it was denoised.
If I can get it reproduce, I'll post the scene, but those frames have re-rendered without a problem.
Could you reproduce the error and check, whether the right click pixel inspector reports a NaN? If yes, this is a major issue and has to be found and bug fixed. So many algorithms after rendering depend on the pixels having a real float value. If it becomes suddenly not a number many problems arise.
Could you render the same error and upload a corona .exr dump of it? Stuff like this has fixing priority.
Disabling denoise is not the solution, as denoise works fine and it just fails at not expecting a NaN as input.