Same issue here - I suspect it may be because we're running LSLib manually. I had to download it manually previously.
I think it's very specifically LSLib since the mod activation requests when opening up a modded save in BG3 asks for many, many instances of one of my mods (5E spells in my case), which is likely interpreted in its domain. Similarly, any attempt to modify the load order is met with the same "Failed to write load order" error.