The official Qt 4.8.5 was built using GCC 4.4. The downloads page lists it as "Qt libraries 4.8.5 for Windows (minGW 4.4, 317 MB)". However, the MinGW provided with Qt 5.1.1 contains GCC 4.8, which is incompatible with libraries built with GCC 4.4. The symptoms of GCC version mismatches are weird crashes, like what you're seeing.
Your options (from easiest to hardest) are:
- Build your program using MinGW 4.4 (Someone is kindly hosting a copy at http://nosymbolfound.blogspot.com.au/2012/12/since-until-now-qt-under-windows-is.html), OR
- Build Qt 4.8 from source yourself, using your version of MinGW, OR
- Port the program to Qt 5