Not sure where you are at, at this point (you will likely get better response time from me if you @ me over on discord).
Yes, deleting the backup folder is generally needed any time you manually copy any file into the strive game folders. Although for the slave list mod I only have you copy in completely new files (the mod system ignores new files otherwise). Note: if any mods are active when backup folder is deleted, those mods can never be un-applied or applied in a later order. Basically when you delete the backup folder you are telling the game that the state it is in right at that second is the "base" version all things should build from or return to.
So... you installed new. Copied the files into the game folder (before running the new install). Then you applied my mod (the slave list mod). Did you also reinstall random portraits?
Other than that, you installed and applied nothing else. And now you are getting the error?
(I don't think you actually ever mentioned which patch/mod of mine you were trying to install. So please include that info as part of what you are attempting)