Not enough tin or coal. Seriously the entire continent I am on has none of either, I even added an xray mod just to be sure, after mining thousands and thousands of blocks out strip mining, and placing countless hundreds of torches in every cave system and ravine I could find. I'm guessing it has to do with the fact they are 40/50/60+ minimum height and the entire area I am in is all plains, beaches, and flat forests. I have copper and iron all over the place in like 20 different forms, things I had no idea were related to copper or iron are giving me copper and iron ingots, but apparently casserite or whatever is the only thing that gives tin, and I have none of it. I don't even care about the coal, I have resin for torches and wood for fuel, I just find it amusing that I'm like 20 hours in without finding a single piece of it (I have seen a few "small coal ores" way out while exploring, but I haven't bothered mining them). I haven't had any luck in the nether yet either, I am probably going to switch to creative and turn on the xray mod in the nether (if it works) and fly around until I find some tin tomorrow, as I essentially can't play without it and I'm not going to continue stumbling around with basic iron tools for another 20 hours trying to find some legit (I'm not going to use magic or bees, I need a break from magic, hence the modpack choice, and I dislike bees). It says ore generation was changed to not be biome dependent, but it feels like if you don't start near some extreme hills or something you probably aren't going to find the basic resources you need for step one of progression.
Edit: I found some tin in the nether not too far away. Still that's not really even a valid solution since the nether requires steel (flint and steel requires a steel nugget) and you can't make steel without bronze. I only managed it because I got lucky and found a couple steel ingots in a chest. I went back a few pages and found half a dozen people complaining about not being able to find tin.
So yeah, just finished the last quick editing needed due to stuff like broken multiple decomp of items containing non atomic or molecular parts and it's here now. If there are any issues just mention them and I'll try to remember why I did stuff.
Not sure if it's related to your file or not, or if it's repeatable (I'm about to pass out, can't load back in and check, but figured I'd mention this while I remembered it), I was randomly going through recipes in NEI after finding some Barite Ore and ended up on the yttriumbariumcuprate or whatever ingot and when I got to the page showing what it would break down into in the decomposer, my game locked up. The decomposer list was blank, it usually takes like a second for it to populate for other items, it locked up before it could populate. There was no crash, I just had a program not responding thing, I gave it a few minutes before force closing it.
Edit: Ignore that, it was something else.