Project crashes after loading

Sorry to write this thread here, Idk where else to put it (I used answerhub as well).

When I launch my project, the FPS example ma loads then crashes within the first 5 seconds of loading. When I look at the crash log, I get this error:

Assertion failed: ReaderPos + Num <= ReaderSize [File:D:\Build++UE4+Release-4.14+Compile\Sync\Engine\Source\Runtime\Core\Public\Serialization/BufferReader.h] [Line: 53]

I did find bufferreader.h, but have no idea on how to fix this issue,and our demo deadline is nearing. How do I fix this issue?