Messages won't open, citing ContactsUI error

Question

I just downloaded a security update (2014~005), and now my Messages app won't open. It immediately says "Messages quit unexpectedly while using the ContactsUI plug-in." Is there an easy fix for this?
Mavericks 10.9.5

Answer 1

You ran a defective third-party "utility" such as "CleanMyMac" that purports to "clean up" "junk" files. What it really does is to corrupt the operating system.
The first thing you need to do is remove that software according to the developer's instructions. Never install it, or anything like it, again. Then see below.
If you don't already have a current backup, back up all data, then reinstall the OS.* You don't need to erase the startup volume, and you won't need the backup unless something goes wrong. If the system was upgraded from an older version of OS X, you may need the Apple ID and password you used.
There are ways to back up a computer that isn't fully functional. Ask if you need guidance.
If you installed the Java runtime distributed by Apple and still need it, you'll have to reinstall it. The same goes for Xcode.
*The linked support article refers to OS X 10.9 ("Mavericks"), but the procedure is the same for OS X 10.7 ("Lion") and later.