this is the way how i fixed the my problems.
i trashed 2 days work of side quests and rewind to the point of giving the potion for pestilencen quest . whıch at that point i didnt give any potion. so i run to the barn killed the guard marched inside and killed the bandit took the letter closed the door run to the sir radzig and saved there. after that i talked to the radzig first and took the kings silver quest and game autosaved that point . and again i talked to the sir radzig and this time directly gave him the letter not mentioned the plague and end of that cutscene kicked in and completed the quest and gave the next one.
the problem was on my case before finding this i gave the poiton cured everyone barged in to barn talked the bandit end of that conversition the part of the i assume handing me the letter game soft locks itself. stay on loadıng screen al time. ı even tried kock that bandit out and took the letter bring it to sir radgiz in that case cut scene didnt trigger. so thats was my solution