Um... what should I do? I noticed that a mailbox appears to be invisible. It's a custom (CC) mailbox, which is probably the reason... It has functioned normally before, but now there's only shadow of it left. I can't click on it and obviously no sim can therefore use it. I know mailboxes can be tricky and cause problems, so that's why I rather ask guidance here before I start messing around. How can I fix it and make it visible again? Should I place another mailbox, and if yes, how? Thanks in advance once again! (Also, we are talking about regular residental, active lot.)

You could try Master Controller's Object Stats option by clicking on the ground of the Lot then NRaas / Master Controller / Object Stats / Active.

MasterController-Interactions#Object-Stats

That will list all objects on your current active Lot. The mailbox will be shown as Core.Mailbox. Select it and click the checkmark to delete. I think the game will automatically replace a missing mailbox on reload but you can find the original base game one in Buy Mode / BuyDebug / Misc. Objects / Colonial Mailbox.

I would save your game before trying this, then 'Save As' under a different name once you've tried the above. Exit and reload the latest save.

That happened to me too once, a CC mailbox became invisible, and I used Object Stats in MC to delete it and replaced it with a new mailbox (also CC) and it has worked fine since then. Object Stats is very useful to get rid of invisible objects.

Object Stats can also be used to delete certain objects from the whole world, like if you have placed lots of CC of some kind that turned out to be buggy or if you just want to delete them all for whatever reason and don't feel like go look for all of then individually. Just removing the installed CC from the folder would replace the placed objects with something else, which is annoying.