Reproduced on recentrly updated version 1.70.

Opted out of 'legacyports' branch to main.
Nothing changed on my side. Not a single file in Arma 3 directory was updated since January, 2nd.
Naturally error 'Bad version 72 in p3d file ...' still persists.

Here's my crash report, hope that helps.
Initial state: Arma 3 is opted into legacyports branch.
Actions taken:

1. Shut down Steam
2. Clear Steam's depotcache directory. (I was suggested to do so for more purity of experiment, shouldn't hurt anyway)
3. Start Steam
4. Opt out from legacyports branch back to main branch.
5. Download required files by Steam.
6. Run Arma 3 => CTD with same error "Bad version 72 ..."

Workaround: go to Steam and opt Arma 3 into legacyports branch using Arma3LegacyPorts as password. This will redownload Apex expansion in suitable format.

Shields, can you imagine a gun designer IRL saying something like "Nah, shooting is only slightly more difficult with the suppressor..."? Nobody would use this gun. Ever. And this is what you suggest, to not use these gun+silencer combinations in game.
Sorry, I can't assume your video as a valid argument. Real game is much more random than shooting targets in predefined locations. Pre-aiming will almost never work (as at 0:26). And btw your aiming slowed down significantly on moderate distance (0:53). Not saying targets were static and very contrasty to background.
As Lex stated, player might be forced to use some strict combinations by mission. Can you kindly ask all mission authors to fix their missions and prohibit those combinations?
EDIT. Ticket author's problem is those gun+silencer combinations are obviously not usable or of limited use.

Seems like AI counts only amount of magazines, belt boxes in case of MG's.
Also, when player takes Mk200 from some box, if there's enough space in vest and backpack, 4-5 boxes of ammo will be taken, which is usually too much. Regular AAF Autoriflemen have only 2 additional boxes.

Could not reproduce in 1.46.131127.

Aaaand you even don't need two backpacks. I reproduced issue this way:

1. Get an empty backpack.
2. Kill an enemy with backpack and open its inventory.
3. Put your empty backpack to the ground while viewing deadman's inventory.
4. Rightclick on deadman's backpack to move it to your backpack slot. => You now have enemy's backpack, AND enemy still has this backpack.

1.44.130654
I had only one mod, and it was disabled.
I ran game as "Play unmodded" from launcher.
I completely removed my only mod and ran the game again ("Play").

All three times I was able to reproduce issue. Well, I don't know what to do now.

Seems like it has nothing to do with placing backpack into another backpack.
'New' steps to reproduce:

1. Get an empty backpack.
2. Kill an enemy with backpack and open its inventory.
3. Put your empty backpack to the ground while viewing deadman's inventory. Close inventory.
4. Get another empty backpack somewhere else.
5. Open deadman's inventory again. Now situation is the same as after unloading backpack from backpack in previous repro-list (as after 3).
6. Rightclick on deadman's backpack to replace your backpack with this one. => You now have enemy's backpack, AND enemy still has this backpack.

Adam, can you please take a look at this issue again.
Thanks.

PS. Should I update steps to reproduce field in issue description?

Hello, Adam. Double-checked it in 1.44.130654, the issue is still present. To put backpack into another backpack you need to drag-n-drop backpack from ground (or any container) to your backpack space, not your backpack inventory slot (just like in the video I provided link for; though the video was recorded yesterday in 1.42.130340, I recheked it before even posting Issue). Hope that helps.

Fully reproduced in 1.46.131127.

Not reproduced in 1.46.131127, but I'm still able to do following:

1. Have full backpack in player's backpack slot.
2. Have empty backpack in full vehicle (not enough space for full backpack).
3. Exchange these backpacks => full backpack stored in vehicle though there were only space for empty backpack.

Whoa, I can put multiple backpacks into backpack since 1.46.131127!