Say, I've had a problem with an early quest.
That quest involves gathering blue and red herb.
But after I picked the herb, I couldn't talk to Mona anymore, even though I needed to talk to her to confirm that the quest was complete.
There shouldn't be anything that prevents interaction with her after gathering all the required herbs. As you said, it's part of the quest. Make sure you are clicking on her with your mouse or interacting with the table directly in front of her with the action button.
If it doesn't help, send your entire save folder zipped to yeehawgames@gmail.com and I'll check it out.