PLL usually means Phase Locked Loop. It is used in tuning in radios and keeping them locked on a frequency. I would talk to Sony. This may be just an indication of a weak signal where the receiver had a hard time locking on to the transmitter. But it might also be an indicator of a malfunction in the device, which is why I would check with Sony.
Remove power for 10 minutes or so, and try it again. If the error message persists, contact Sony. Sony has a factory service center in Oradell, I used to take our BetaCams there occasionally. I think it's still there, whether they repair wireless systems I don't know.
Thanks. Sony Teaneck service is walking distance from me so I can have them check it out. My interview subject had dropped the transmitter into the cushions of an easy chair so perhaps that's why the receiver was having a hard time locking to signal.