Final answer:
Mme Loisel's problem upon receiving the invitation was her dissatisfaction with her wardrobe, feeling she had nothing suitable to wear to the event, which led to a major conflict in the story.
Step-by-step explanation:
The problem Mme Loisel faced when her husband received the invitation was her perception of lacking appropriate attire, which made her feel inadequate for the event. Despite her husband providing the invitation to give her pleasure, which he thought would make her happy, she reacted with discontent because she had nothing suitable to wear. This became a major conflict in the story, showcasing the themes of vanity and the pursuit of status.