Additional Placement overrides the block placement system to allow rotational blocks in all 6 sides of a cube. with DoubleSlabs installed placing a slab onto its side via A.P. causes the game to crash.

You can temporarily "fix" this by editing the config for Additional Placements to disable its slab placement function, it's the one that directly references Double Slabs in the description/tooltip. Personally I also set Double Slabs config to disable automatic placements and make it crafting only for vertical slabs.

I'd also like to ask for support for the mod though since it would be very nice to have both.