Compare this list (client-side-only mods) with your mods:
https://www.dropbox.com/scl/fi/yeldxfv8ed60e4uflc2fi/Client-Side-Only-Modlist.xlsx?rlkey=8376c5bk7b33je2tad4p3ybrg&st=qf6osvit&dl=0
There is an invalid entry in your level.dat
Create a copy of this world and open it in singleplayer - if this works, upload this world to the server
If not, add the latest.log from the singleplayer test
Also make a test with another Launcher like MultiMC or AT Launcher
Search for the Modpack and install it or download the .zip file from curseforge and import this modpack