The problem is solved. I'm still not 100% certain what you mean by 'rebuild map cache', as I found no clear instructions for CK2 in particular through google, but I'm pretty sure I figured it out.
I deleted all the files in the My Documents/Paradox/CK2/gfx/flags folder, as well as all the files in the My Documents/Paradox/CK2/Maps folder. Regardless of if this was exactly what you were talking about, that fixed the problem. The game crashed on startup when I first started playing after reinstalling windows not long ago, and my guess is this was where the bug was introduced into the files that have now been replaced.
Thank you. CK2 can be buggy at times, but this kind of support is what makes it, and the DLCs, worth it anyways.