Hmm, maybe try running it without Optifine installed and see if it still happens? Can't see anything else that would be causing a problem. Usually, the stacktrace points to a particular mod involved in the crash, but that report isn't really giving me any clues. Unless you know which one of your mods writes NBT tag data to files?
Can confirm I tried to pregenspawn 100 and it crashed with Optifine on the server and after removing it there was no issue.