I'm guessing you've got bad data in your group of captured slaves. This could both mess up the control panel (travelling outside) and throw out this error message when trying to move your captured slaves into the mansion.
Keep a copy of the bad save file set aside in case someone wants to look at it (or edit it). I suggest you reload from before this error for now. Do you know how you might have got bad data into your list of captured slaves?
I don't have a save from before the bug, but Im not too attached to the save and can just restart. As for a cause it might be that use invigorate to push how long I was out in the wild. One of the days I did before the bug happened I made close to 10k by using the mark spell to teleport to mountains, catch a bunch of harpies, then use gorn to teleport back to mansion, use the learn button to trade skill points for mansion upgrade points and sell the harpies, using invigorate and sedation when needed. After a couple days of doing this I rushed through the entire Cali subplot, part of the main plot from the get laboratory to handing over the dark elf, and and the yiris challenge until the need for the alchemist, I also bought a bunch of upgrades for the mansion before calling it on that single day. I noticed the bug a couple of days later when I wanted to change the mark location to behind ambergaurd to hopefully be closer to the where I needed to be for the plot, I thought it was just I couldn't use spells back there but when it persisted to no matter where I was I knew it was bug.
Edit: some grammar because I can't spell apparently
