Ajax Error 500 on Import

I know there have been mentions of an AJAX 500 error plenty of times, but looking through everything, I don't see any that quite matched this (most have been fixed and the fixes already patched).

CSV file with pipe delimiter. Nothing complicated. Exact same file worked earlier in the day. Deleted the nodes and reupped. Started getting this error and now nothing past the initial "25 percent" will import.

An AJAX HTTP error occurred. HTTP Result Code: 500 Debugging information follows. Path: /batch?id=119&op=do StatusText: Internal Server Error ResponseText: {"status":true,"percentage":"25","message":""}

Update: ALWAYS errors out at either 50 or 100 nodes.