I think… I think I’ve fixed it. 
Turns out you can set a minimum amount for the Lightmaps and a Maximum. Or at least, you can set something in the Min and then in the Light Map Resolution under General Settings in the mesh viewer. So the min is 128 and the one under General Settings is around 256. Seems like a sweet spot. Either way, the weird black patches and seams are gone. ^^
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