It could be related to having Enhanced Slave List as I can’t speak to the compatibility, but that almost looks like its an item that cant determine how much it weighs. Does that slave have any items equipped?
So loaded up the game to look and see if it was related to weight and noticed that on my most recent save she now seems to work fine. If I add her to my party I can leave and use my inventory as normal. However, when I go back to any older save she is still broken and can't be used. She has no items in either save.
In the older save I looked into it and the issue definitely is related to weight. When she is in my party I can't add anything to the inventory, it just gives another error message:
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If I try to remove something from the inventory the item will be removed but the weight won't decrease and the same error will occur show in the log. It will either display the old weight or no number at all. Once I swap her out of the party my inventory corrects itself to the correct weight.
During this process of playing around for a trying to figure out why she works in my most recent save and not the older ones I seem to have found a way to fix the issue, in game at least. If I add her to the party, go to the inventory screen, select the slave causing the error, and then switch from mansion inventory to party inventory or vice versa, the inventory weight will correct itself and she becomes usable in a party. Fixing her like this once seems to work permanently (or at least for a long time, she still worked after being removed and re-added to the party, even several days out of it).
Yes. It's working fine now.
I still don't know what caused it or why only that one slave. I even tried getting other slaves to similar stats but they weren't broken like her. I thought it was because she had high strength (5), but I made other slaves with the same strength and they weren't broken. But at least it's working now.