The simple answer ... too low of a setting for render. Before someone comes on with all the technical stuff and settings. Try some of the higher end settings in the render settings panel especially more passes. It also has to do with now many mesh lights you have. But settings are you best starting point. Don't get me wrong. The settings and techniques are important ... very important. I just want to give you a spring board first.

Boni

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"Be Hero to Yourself" -- Peter Tork