They developed nice flower buds a month ago, but did not open.
Instead they became ugly.
I sprayed with all the solutions I had: copper oxide, systemic, etc.
I gave them some compost manure in spring, and then gave them MiracleGro every 2 weeks.
I also provided water. What is the problem, please?

Probably too much fertilizer and lilies are not fond of manure either. Possibly too much water too but you didn't say how much water you gave them. Fertilizer should have a lower N number than P and K. I use 5-10-10 granular tomato fertilizer sprinkled lightly around the emerging "lily nose" in the spring then no more until bloom is finished.

I also find oriental lilies a bit more difficult than most other lilies, for example Asiatics, trumpets and orienpets.

I would remove the rest of the buds but none of the green leaves then see how it comes back and grows next year. Hopefully the bulb hasn't rotted due to too much fertilizer and/or water.