This is the solution that works for 90% of affected users. Black Ops 2 released in an era where dual-core processors were the norm. Modern CPUs with 6, 8, or 12 cores can cause the game’s thread scheduler to break, leading to the "default asset" error.
This is a well-known PC error, especially for the Steam version. Below is a concise “good paper” (i.e., a clear guide) to fix it. call of duty black ops 2 could not load default asset