So I figured it out, the error is caused by having "Hide inactive ESPs/ESMs" enabled in Workarounds. Simple...
After creating a whole virtual machine trying to replicate the error, I installed LOOT 11 again and looked at the logs. Instead of just giving up breaking like LOOT 12, it spammed this a couple hundred times: [17:51:23.173766] [debug]: Checking if plugin "HighResTexturePack01.esp" is active. [17:51:23.199791] [debug]: Checking if plugin "HighResTexturePack02.esp" is active. [17:51:23.226817] [debug]: Checking if plugin "HighResTexturePack03.esp" is active.
That gave me the clue. After that LOOT 11 just displayed the plugin list and I could sort, but probably slower than normal as it continues to panic about the missing plugins.
I can't remember why I needed to enabled that. I searched a bit but couldn't find much information besides the STEP MO Guide saying "Currently bugged, so do not use." I'm guessing that's what you would advise?
