A BoA Boliviana de Aviacion Boeing 767-300, registration CP-2880 performing flight OB-777 from Madrid,SP (Spain) to Santa Cruz (Bolivia), was enroute at FL310 about 130nm southwest of Madrid when the crew drifted the aircraft down to FL250 and returned to Madrid for a safe landing about 80 minutes after departure. On Jun 9th 2019 the passengers staged a protest at the BoA counter at Madrid Airport demanding to be taken to their destination in Bolivia. Later the day the regular flight OB-777 of Jun 9th 2019 departed Madrid on time (no flight had been scheduled for Jun 8th 2019). On Jun 10th 2019 the airline reported an engine (CF6) ingested a foreign object prompting the crew to return to Madrid. The occurrence aircraft remained on the ground until Jun 10th 2019 and departed for the flight OB-777 of Jun 10th 2019.