Yuta and I noticed that there was a funny glitch frequently (maybe once a minute or so) in the PRX signal as well as the IMC transmitted power. It turned out that this was due the ISS keeping trying to acquire the locked loop with no success. According to a 2 days trend (see the attached), the diffraction power dropped quite rapidly yesterday and eventually plunged into a so small value that the ISS can not lock itself any more. Because of that, the ISS had been down from 22 pm local yesterday.
I increased the offset in the error signal (H1:PSL-ISS_REFSIGNAL) from -1.85 to -1.75. This brought the diffraction power back to the nominal of 10%. Now it seems running fine without a difficulties in locking it.