Using adventurebackpack-1.7.10-0.8b.jar and forge 10.13.2.1291, it seems that theres a texture linking problem on your standard backpack as it not finding the texture for it (see the screenshots below).

This is only on the Standard backpack btw, the rest are fine.

Same thing happened to me after a few days. I removed the mod, downloaded a new copy, and added it back to no effect. In the NEI list of items, two are pink:

• blockAdventureBackpack.name (#0165) 165
  adventurebackpack:blockBackpack
• Adventure Backpack (#4100) 4100
  adventurebackpack:adventureBackpackStandard

It's a missing textures problem. I purposely left some textures "for later" and then life happened. Always finish your homework, kids.

It'll be fixed in next version, no excuses.

With me too :/
This happerns after a little time running the mod!