As you pointed out Dwarden, the description header thing may be a possible cause. The base template also suffers from the same issue. (switching to any role that's not civilian/zeus).
Template:

I tried running the Ivachev mission by slotting a civilian, it loaded fine. However, the moment i got into a vehicle, the game locked up. Didn't leave a crash dump, but here's the RPT file.

EDIT: Tested again, as long as i'm a civilain and not in a vehicle it's fine.

I removed the 3 main F3 components - init.sqf , description.ext, briefing.sqf. Still crashes the same way, but doesn't complain about the header in the same way.

Okay, so even removing *all* F3 components didn't help in the original mission, however trying a clean mission works (i.e. allows me to slot).
e.g. of clean mission:

However, x64 still causes crash when entering vehicle as driver (passenger slots unaffected). x86 doesn't cause the crash.

More dumps and logs for crash on slot screen issue:

Removing other side slots except OPFOR causes crash as soon as i press "play now" in editor.

19:18:07 Client: Nonnetwork object 7888ecc0.
19:18:07 EPE manager release (0|8|0)
=======================================================
-------------------------------------------------------
Exception code: C0000005 ACCESS_VIOLATION at 01AC85E0

As you can see, no mention of description.ext::Header.

Tried binarising file, same effect.

So apparently, if i make all the units unplayable, and create a new playable Red Army unit, it works fine. But the moment I make the pre-placed units playable, it can't load into the slotting screen and crashes with the same error.

I don't know what the problem is, but I'm out of ideas on what to test now, so i'll stop.

Fixed on latest performance branch as of the time of writing!
Thanks @Dwarden and co. !