Status: Fixed for next update. Note that it is impossible to tell if any given level is "normal dirt style" or "Genesis dirt style", so it will load it into the standard version of the Dirt style; this is not fixable.
I had this issue for a long time, but I did not post about it because I thought it was a problem I caused myself with my own meddling. But now I realize it happens on both V1.38n-A and 38n-B [and a few older versions I think. Unfortunately I don't have older versions at present so I can't test]. I altered nothing with the latest editor.
When I try to open a dirt level from Genesis Mega Drive the list index out of bounds message appears with the level in total thin white diagonal lines. This exact issue also happens if you try to open a level with terrain pieces for which you don't have installed in your graphic set. At least this is how it used to work.
I can open any other dirt level just fine.
In case you don't have this I uploaded the levels; these are all of the genesis exclusive levels which ccexplore or somebody ripped from the game and converted. As far as I know they are in the standard Orig old Lemmix style format. All the rest of the levels can be opened just fine.
I tried using old Lemmix to save one of these dirt levels in custlem style but it still couldn't be opened.
My first guess to this problem is that there are somehow more pieces or something present in the old dirt style, but this doesn't make much sense. The new NeoLemmix styles have more objects. The terrain peices are the same at least; I just checked.
They work fine if I try to open them in the "Custom Lemmings" (aka traditional Lemmix) style. Not so much if I try to open them in the NeoLemmix style.
This bug seems to happen for all traditional Lemmix levels that are in the Dirt style (Genesis or not) when attempted to open them in the NeoLemmix style. I'll look into what's causing this.
In the meantime, there is a workaround: After you load the level and it gives the error message, open the Level Properties dialog (the one where you set the title, lemming counts, etc), click the "..." next to the Style box, and choose the correct graphic set from the list (NeoLemmix -> Dirt (Genesis)). Once you do this, the level will show up properly.
ok thanks --- no idea why I didn't think to try that myself :-\
Fixed for next update.
Do note that the fix will result in loading it into the normal (non-Genesis) version of the dirt file. This cannot be avoided, as there is no information stored in the level file as to which one is correct. It's probably still better than not loading it at all (or at least, seeming to not load it at all).