Thank you for your quick response! I'm sorry about the log, I moved it to pastebin. I downloaded AnimationAPI and put it in my mod folder for the modpack, but the same thing happened: it launched, loaded for a little, then reverted back to the launcher with no error message.
If I had to hazard a guess, I would say that maybe the AnimationAPI mod isn't registering in the pack (if that's something that happens)? I have it in the folder, as I said, but I think it says it's not there in the crash log. Here's a link to the most recent log and crash report, as well as a screenshot of my mod folder.
https://pastebin.com/bVkqXSuG
https://pastebin.com/TL6U5bBf