When this problem occurs?

If you have installed a custom rom on your LG G2 probably the DPI setting has changed.
The DPI setting is used to calculate the pixel density, and consequently the real measures of your screen. Google Cardboard calculates the distance between eyes using the DPI setting, and if it's wrong some apps using the old CardBoard SDK don't show properly.

Here's an example

With 480 DPI (CyanogenMod default setting) With 424 DPI (LG G2 real DPI)

How to fix it?

Please choose the method that you prefer

Method 1

Fixing the problem for some apps, keeping your current DPI setting.

1. Open Cardboard's app
2. Tap to Settings
3. Tap to Change
4. Scan with your camera this QR Code:
   WARNING: If you do this you can't go back to the default settings!
   
5. Enjoy your cardboard

Method 2 A

Fixing the problem globally changing your current DPI setting.
This method is perfect for CyanogenMod based ROMs.

1. Go to your phone's settings
2. Tap to Display & lights
3. Tap to LCD Density
4. Change the value to 424 DPI
   WARNING: If you don't find this precise value, higher values are better than lower ones!
   
5. Reboot or soft reboot your phone
6. Enjoy your cardboard

Method 2 B

Fixing the problem globally changing your current DPI setting through an app. [ROOT]

1. Open Play Store
2. Find and install Texdroider Dpi
3. Open it
4. Grant root access to Texdroider Dpi
5. Change the value to 424 DPI
6. Tap the Apply button
7. Reboot or soft reboot your phone
8. Enjoy your cardboard