Could you please give us an option to abandon quests? This could help with restarting, or getting around bugged quests, or at least getting the bugged quest's marker off of your map.
I'm no dev, but abandoning quests you've completed would allow a function for players to repeat the quest in question and gain a second reward. However- it would likely fix people who can't complete the quest...
Unfortunately the only option otherwise is to check with the Old Quest Maker Fixer near the train station in Cantermore or to talk with a dev if you catch them around. What quests are you having trouble with?