...and another tileset without translation table in the NeoLemmix data folder .
To my knowledge, Sandopolis was the last of these graphic sets also created for Old Formats.
I didn't have the translation table, so I created it anew via re-converting the tileset, then deleting my conversion for the sake of the original.
All Pieces had been renamed, though (changed nomenclature from "Terrain Piece 3" to "terrain_piece_03", for example), therefore NeoLemmix couldn't find the matching pieces even though they were present in the folder.
Therefore, here's my edited version of the translation table. I was able to convert all my levels featuring the Sandopolis tileset with this; please report if I've missed anything.