It’s possible that the newly modded version became corrupted. I would try modding it again or even try with a different save to see if it still happens. I use the Gibbed BL2 editor often so I know what you’re attempting to do. You should also try running Horizon as administrator.
Did you unpack the .dat file. there is a .dat unpacker that lets you edit the file with out corupting the file then you can repack the file and re-insert it