@mbaluta Thanks for getting back to me!
Opera Version: 31.0.2254.112029
Phone: Galaxy J7 V (SM-J727V)
Android Version: 7.0 (J727VVRS1AQJ2)
Note: the error spontaneously disappeared for the past week, then reappeared today. I think that some system file is being treated as garbage, because it returned today when my phone warned me that my storage was running low, as it did last time when it started.