fm output runs at 192 kHz so that's why it doesn't work.
For me, in the new GUI at least, FM Output works on my 48khz ASIO card in Win64. It's only when I try to enable the FM Transmitter that it gives me the error about needing the higher sampling rate. This is fine because I don't have an FM license or need any FM processing.
Am I the one experiencing the unexpected behavior?
I think it's handy as heck to be able to use 2 outputs (headphones and speakers), in addition to being able to stream via Icecast to my phone, without needing an audio device capable of 192khz. I hope this behavior is standardized across all versions.