Thank you for reporting.
I was not able to reproduce this issue with the reproduction steps provided. I was able to accept and start other tier 6 quests after failing the first tier 6 quest I accepted.
Please provide your map information and the location of the trader that you had the issue at.
Your map information is located at, AppData/Roaming/7DaysToDie/GeneratedWorlds/New Gobozovi County/map_info