that´s a bit strange, try to create the RT for the hdr_mtl on your own like this (btw standard antialiasing doesnt work with pp effects):
Click to reveal..

function main()
{
if (d3d_shaderversion<2020)
error("Shaders require Commercial Edition!");
//d3d_antialias = 4;
//video_set(1920,1080,32,1);
video_set(1280,720,32,0);
fps_max = 60;
// load a level, and create sky, water, and a model :
level_load("small.hmp");
ent_createlayer("skycube+6.dds",SKY | CUBE | SHOW,1);
//ent_createlayer("555.tga",SKY | CUBE | SHOW,1);

//camera.fog_start = 10 ;
//camera.fog_end = 2000 ;
//d3d_fogcolor1.red = 255 ;
//d3d_fogcolor1.green = 180 ;
//d3d_fogcolor1.blue = 20 ;
//fog_color = 1 ;


ent_create("water.hmp",vector(0,0,-5),water);
ent_create("blob.mdl",vector(0,0,30),blob);
vec_set(sun_color,vector(200,240,240));

// set up some parameters and a HDR effect for the camera
vec_set(camera.x,vector(-130,0,80));
camera.tilt = -20;
def_move(); // activate flythrough
mtl_hdr.skill3 = floatv(50); // exposure compensation factor
BMAP* HDR_RT = bmap_createblack(screen_size.x,screen_size.y,8888);
PP_Add(mtl_hdr,camera,HDR_RT,NULL,NULL,NULL);
mtl_erode->skin1=HDR_RT;
PP_Add(mtl_erode,camera,NULL,NULL,NULL,NULL);

}


I´m not sure but you probably need another small modification in the pp_erode.fx file:

Click to reveal..

//Texture TargetMap;
texture mtlSkin1;
sampler2D g_samSrcColor = sampler_state { texture = <mtlSkin1>; MipFilter = Linear; };
...

Perhaps you only need to deactivate the AA, just try it.