This happens when Eevee's "Screen Space" effects don't align between the different camera views.

• Reason: Effects like Screen Space Reflections, Ambient Occlusion, and Bloom are calculated based only on what's visible to each camera. At the edges, this creates inconsistencies that look like seams.
• Solution: The best solution is to bake your lighting. Before rendering, bake your scene's Indirect Lighting (with an Irradiance Volume) and Reflections (with a Reflection Cubemap). This creates a consistent lighting environment and solves most stitching problems. For best results, also disable effects like Bloom and Depth of Field.

This is the most common issue. If you save, close, and later reopen a project, you must click the Setup cameras and scenes button again before rendering.

• Reason: The addon needs to rebuild its internal settings in memory, which are lost when Blender closes.
• Solution: Just click Setup again. It's a safe operation that won't duplicate objects but will refresh the addon, making it ready to render.