I am having this same exactly problem as well but it makes me feel better to know this problem is not unique to myself, but is happening to others. I do believe it is indeed the new update that causes this and I hope they fix it soon because I would love to play my mod packs again. I even tried to allocate more RAM and memory to my addons and modpacks and still nothing. I also had a problem when I loaded up a mod pack (Project Ozone 2) and I launched my world, it instantly crashed with the error "Constant server tick loop" and I could not launch that world anymore. I could make a new world and it worked fine until the next time I loaded it. It would crash. Now I am not sure if that has something to do with the new update as well, but I am very frustrated with all these bugs and errors happening to not just me but for everyone else as well.