Hi, there is a way to rename the System folder, whatever is the new name, definitely.
Once the operation's done, no more need to redo it, even if you want to rename the folder again.
1) Go to your System folder you want to make renamable, located in the main Lineage II folder.
2) Open L2.ini with your favorite L2 file editor.
3) Search for L2ShaderPath=..\system\ (under ). Change ..\system\ to .\
4) Search for Paths=../System/*.u (under [Core.System]). Change ../System/*.u to ./*.u
5) Save your work and now, you can rename your System folder without Critical Error, whatever is the new name.