I'm not even sure this needs fixed. Kinda funny to have run into. Double speed belts adjusts belt speeds so that each tier is double the last. When combined with Ultimate speed belts this appears to cause the Longer Underground Belt mod to set the highest tier of belt to an underground distance apparently FAR beyond what the game will allow.

Error ModManager.cpp:1508: Error while loading entity prototype "original-ultimate-underground-belt" (underground-belt): Value (6143.000000) outside of range. The data type allows values from 0 to 255 in property tree at ROOT.underground-belt.original-ultimate-underground-belt.max_distance
Modifications: Ultimate Belts › Natural Evolution Enemies › Double Speed Belts › Natural Evolution Enemies › Longer Underground Belt (Aligned) › Additional turrets

Well was fixing another issue today, so might as well put a math.min on that value to clamp it to 255.

Though if you leave the config option on to add belt costs to the longer range it increase the final tier of belt to 1500+ belts per craft. Will tweak that as well...

Put in a fix. Not sure what is going on with the recipe changes on ultimate belts. It seems to be able to select a belt correctly them when double speed is installed, but can't find them when it is just ultimate belts alone.

Uploaded 18.3 with that one known issue. If i get more time someday i'll try to figure out what is going on.