I did some more test and changing the LDO voltage has similar effect as changing VCO bias current - e.g. I get the the “cannot deliver frequency” error for 100.1 MHz, so I set 1.4069 V. I helps. Than I change the frequency to 200.2 and it fails. I change the voltage back to default 1.2098 V and 200.1 is OK. On the other hand bypassing LDO seems remove the error message in all cases. But as I mentioned this behavior is not stable, so there may frequencies which fail even with this and as I understand this is not supposed to be the solution, correct?