Keita pointed out that the ISS 3rd loop 560 was overloaded, and was in low noise when it should have been in high dynamic range. I went to check it out and indeed, it saturates even without any input or with a terminator.
Fil didn't have a spare working 560, but loaned me an SR650. We are using an AC coupled low pass at 30 Hz, gain of 0 dB, positive polarity, to imitate the settings Keita used in 27895
We don't know how long the 560 has been overloaded, but this probably is the reason why we have had so much difficulty with the CSOFT instability in the last week.
First lock attempt after Sheila's fix we were able to go to 50W without any instability. No fancy offsets or anything, so the PRC gain still dropped (to ~24), but we didn't have any trouble acquiring or holding the lock.