Apple TV 4k is constantly resetting my Sony TV

I have an Apple TV 4k and a Sony KDL-40WE663. The Apple TV is connected with an HDMI cable to HDMI2 on the TV. Everything worked OK for two years. Suddenly, recently, every time I turn the Apple TV on and switch the TV to HDMI2 the Sony TV gets itself into a loop of constantly powering off and powering back on again. The Apple TV's home screen is never displayed. I've tried using a different HDMI cable (same behaviour), I've tried connecting the Apple TV to a different TV (everything fine), I've tried plugging something else into HDMI2 on the Sony TV (everything fine). I've also been into the Sony TV's BRAVIA Sync Settings and turned BRAVIA Sync Control OFF, which made no difference. The Apple TV (tvOS 15.5.1 (19L580)) and the Sony TV (v8.616-1000-2.750-0002) both have the most recent software installed. Can anyone suggest what the problem might be and/or what to do about it? Thanks