What is a single word/phrase that can be used in place of 'mistaken assumption' or 'incorrect belief'?

What is a suitable word that can be used in the below story scenario?

I’ve considered the following words; Delusion, Conjecture, Fallacy, Belief, Erroneous assumption. Is there a better word anyone can suggest?

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In a small village there was a girl afflicted with a mortal disease. The medicine to which was expensive and could only be obtained from a doctor who lived seven days journey away.

Her family had the money, but no one to make the perilous journey.

A man, her lover volunteered, and promised to return with the medicine. He got the medicine, but on the way back was mauled by a bear and succumbed to his injuries.

His absence caused the village folk to gossip that he had taken the money and ran away.

The girl’s condition worsened, and at soon she passed away under the (mistaken assumption/incorrect belief) that the man had tricked her and ran off.