I currently have two different versions of DF on my computer; one is an older version that's mostly vanilla while the other is d16 with Dig Deeper, some of the Civ Forge, and the Primative Civs races added in. The latter worked fine for a few days, but has started doing this somewhat randomly.
On the chance that something had been corrupted, I deleted off everything from the DF16 folder and did a fresh install before replacing the files that I had used before. No dice as it's still doing the same thing.
The errorlog lists several problems:
Duplicate mithril entries (there is only one entry under metalgloss though I need to check the Reaction file to see if there's a duplicate under that.)
Sugar/fire/colossal/monstrous beetles are all listed as having duplicates.
Emerald/Giant Wasp are listed as duplicates.
It also lists some unknown Spheres, but I can't find a single creature with just Plant or River listed as a Sphere so I don't know where that's coming from.
Out of all the folders, only the original Insect (ordinary beetles), Insect_Extended (one each of those listed above), and each of the language files (words for beetle) have any instance of the word beetle in them though I did notice that one of the beetle entries had the same listing under the offspring category as another, which I fixed. So there aren't any duplicate entries of those. I did turn out to have a duplicate Goblin language file in there, but deleting it didn't fix anything.
One other note: the mithril error popped up early on and was indeed a duplicate entry, which I fixed. For a while, it didn't list that under the errorlog but has started doing so again later on despite the fact that no such duplication exists.