EDIT2: look at my 2nd post below for the cause

In fact I have 2 of them (both of the same turn essentially)

fleets bugged.zip

AutoSave 311.zip

I had intented to quickly save the game and reload the same save because I was having trouble with a bugged fleet but it wouldn't load at all, so that's quite an issue.

edit: 

the last save that does load: AutoSave 307.zip

the first save that does not (8 minutes later): AutoSave 308.zip

some of the things I did that turn:

  refused peace offer and the forced peace

  declare war on red to start sieging his last planet

  started blockading and sieging the unfallen homeworld who was gonna win the competitive event for most food produced

  started making moves on investigating the endless factory (craver quest chain)

  guarding the Mrigashi system for "the living plauge - part 2" quest

  gave vacate order on Spica

I suppose any one of these or something else entirely could be at cause

I went ahead and did some testing and I found out the root cause

first check out this working save file: test2.zip

case 1: 

   do nothing except press the end turn button and let it resolve

   try to load the autosave file that gets generated at the start of the next turn. it fails

case 2:

   Go to system Spica and cancel the vacate order

   end the turn. the game will warn you that there are systems without build orders and it will list the Spica system twice. (this may be a separate bug)

   ignore the warning and end the turn anyway and let it resolve

   try to load the new autosave. it will work

extra notes: 

  abandoning the Spica system a turn later gives the same 100% reproductible corruption, so it doesn't seem to be a conflict with an event etc

  abandoning a different system gives no problems.

  spending a few more turns to manually sell off all improvements and eat the 2 non-craver pops did not offer a work-around. the save after vacating the system still doesn't load

Mmh good investigation work, I'll see what I can find on my end :)

Cheers!

Edit: Yeah I confirm your findings, the errors I'm getting are pretty vague though - I'll hand it to our programmers for further investigation.

Edit for other Devs investigating the issue: Here's a ready-to-load save that produces the aforementioned errors.