Welcome to the SeeMeCNC Community

So I'm running a Rostock V2 with and E3d v6. I recently installed a second 30mm squirrel cage layer fan (two layer fans total). During the process I accidentally knocked out my thermistor quick-connect plug at the hot end, enough that the signal was lost, but I pushed it back together and it seemed fine.

Fast forward to the first test print with my new double fans of doom, and everything seems to be fine. A couple of layers up (or sooner, on subseqent prints), I notice the extruder has stopped and the LED is reporting "def" for both the bed and the hot end.

So, does this sound like a wiring/resistance issue? I pulled on the thermistor wires while at temperature and had no change, but its possible my crimping game is not on point. Could it be a power issue with the new fan? The fact that it stays at temp until the printing starts, and that its the both hotend and bed going "def" is bewildering and I just don't have the time right now to adequately troubleshoot.
Any ideas/starting points are appreciated!

It's definitely a loose thermistor connection. The firmware will halt and display def if it can't read one or both thermistor values.

I've seen where this can be intermittent. My machine would start a print just fine and default a few layer later like you're describing. Reterminating the hot-end connections did the trick. I was also (stupidly) using solid core wire because my stranded spool hadn't shipped yet. Changing to wire that's designed to flex and be crimped did the trick.