Hey Lithium. Welcome (I'm fairly new here too)!
Though I've never downloaded the starter packs here, it looks like there's a problem with the sprite file (the .sff file) of that particular stage. I'm guessing the .def file (the one that provides the data of the stage) has the wrong location for the .sff file and can't find it, which crashes your game. If you don't mind missing the stage, the quick fix is to go into your select.def file and locate the stage (you can just CTRL+F NBC_KurukuruD4). Add a semicolon (';') in front of the stage name, which omits the stage, or just delete the line that contains the stage entirely.
If you'd like to keep the stage, feel free to PM me and I can walk you through the steps to fix the problem (or you can just send me the stage file and your select.def file and I'll try to fix it myself). Good luck!
EDIT: As for the missing character sprites, it's likely they're under the wrong portrait group or something. Usually, it's 9000,0 for the select screen portrait and 9000,1 for the larger one, but some mugen creators like RMDB have specialized ones that use 9000,100 or something like that instead. That's probably the case, and if it bugs you, you'll need to go into FF3 and change the portrait to the right group. More unlikely is that the portraits are off-aligned or don't even exist, but that's pretty rare.