Recently every time one of my sims tries to use the register at the consignment shop in Twinbrook to consign paintings, she resets to her home lot. I get an "index out of range exception" script error. Apparently the "array index is out of range"? (This started happening just today; she tried to use the register twice, and I got two identical messages. I only uploaded one, as they are identical.)

How serious is this? Is it just annoying, or a sign of something worse? How do I keep this from continuing to happen? Delete and re-add the register? (This is very annoying as she's a self-employed painter.)

Hmm. I think we're going to need a coder to explain that exact error. Doesn't look seriously game-breaking to me, but it is if your sim requires sales by consignment to make a living. But in the meantime, a few notes/questions:

-- Do you have any tuning mods from another source that affect consignment or the value of paintings? If so, one of them might be acting up.
-- Your sim has a Consignment Skill of 0. Has she ever successfully consigned before? You might try using MC Cheats, which you already have, to bump her skill up to Level 1 and see if that makes any difference.
-- Have you considered trying the Consigner mod? It has a reputation for fixing consignment related errors, but I honestly don't know if this is one of them. You may want to look at the mod's FAQ first though, to see if this is an addition to your game that you would really want.
<a href="http://nraas.wikispaces.com/Consigner+F ... ner+FAQ</a>

I believe installing Consigner will fix that, the game doesn't cleanup properly all the consigned items and that angers the interaction when it's trying to retrieve all the stuff that it's going to show you in the dialog.
Consigner has a built in cleanup process for just that, so my suggestion is to refer to igazor's links above and install it if you use consignment a lot

I do not have any consignment mods, or affect the value of paintings.

I'll take a look at Consigner. (She has successfully consigned a number of paintings; she's sold most there, and she's level 8 now, I think.)

She is at Level 8 on both Painting and Sculpting, but I didn't mean those. Consignment is a hidden skill, one gets better at selling stuff with practice. She is at Level 0 on the hidden Consignment skill, which can be reached with MC > (Sim) > Advanced > Skill Levels.

But as JunJay, who is a mod developer, has noted there appears to be a more likely cause of the issue.

I've installed Consigner, and hopefully that will fix it.

Works beautifully now. Thanks very much!