Richard warned me that the Fiber polarization had probably drifted at EY due to some work done around the table. Indeed it had drifted to up to 33% for the wrong polarization. However, Stefan had increased the threshold to 36. I returned the threshold down to 30 and adjusted the controller. I also had to adjust the EX polarization when I turned the controller on, which forced both polarizations to drift. All is good now.