KFF checks the clock frequency before starting emulating a CRT. This is done to prevent the PAL firmware to start on NTSC machine and vica versa.
If the U64 does not supply a clock with a stable frequency during startup, this could explain the issue.
Could this check be removed to make it compatible to the Ultimate devices then?
In order to make the 1541 Ultimate II(+) work with the Ultimate 64 (not really neccesary, but who cares?) I implemented a workaround in the firmware:
At poweron, wait for IIRC 5 seconds for PHI to arive. If it does, all is well and waiting is aborted. If not, go on - the U2+ has a standalone mode...
Perhaps such a waiting for PHI2 can be done in the KFF, too.