1. The very first thing to do if you run into issues with Long War 2 (or any XCOM2 mod for that matter) is to backup and delete the following folder: \My documents\my games\xcom2\xcomgame\config. This is commonly refereed to as "refreshing the ini files"
2. The second thing to do is reboot your computer.
3. If you are running XCOM2 with only LW2 installed, then try unsubscribing and re-subscribing to LW2
4. If all else fails, unsubscribe to LW2, go to Steam > XCOM2 > Properties > Local Files and click "Verify Integrity of Local Files", and then re-subscribe to LW2.
Trouble with Additional Mods
If you are running other mods in addition to LW2 you can try these troubleshooting tips in the event of a mod conflict.
1. Uninstall the most recent mod(s) that you've installed, or the mod that was updated most recently (sort your Steam Workshop by "Subscribed Items" and "Date Updated")
2. Install and run the "Remove Missing Mods" mod. After removing ANY mod open a recent Avenger save and click the "Unbind Missing Mods" button, save the game, exit to the desktop and reload the game. This should remove all traces of the now uninstalled mod(s).
3. Some tips from the Steam XCOM2 workshop.
4. Some tips for Mod Users from Grimy Bunyip.
2K Troubleshooting Links
GPU performance improvements and troubleshooting
The "Pure Virtual Function . . ." Bug
This bug is directly related to your system memory (RAM) being overloaded. It appears to be rooted in the Unreal Engine code that runs XCOM2. THERE IS NO FIX other than reloading an earlier save. It is not related to a single mod, and is usually a combination of mods or other programs (or even Windows processes) causing your system RAM to max out. If it happens repeatedly begin by unsubscribing to some mods (Voice Packs in particular)and try running the game again.
The "Stuck Under The Avenger" Bug
This big is most likely caused by an overloaded Graphics Card, it has nothing to do with the "power" of your GPU and can happen to anyone. It appears to be a timing issue when several functions cause your GPU to freeze up. The on screen location that you see under the Avenger is allegedly the map location "0,0" and it appears that the game defaults to that location when your GPU cannot process the incoming data. Reducing the number of mods appears to help, but as with the previous bug it is not caused by a particular bug, but it apparently exacerbated by running mods that tax your GPU.