When you use the debug breakpoints, the peripherals still keep running, this will cause unexpected behavior in projects that use interrupts, timers, low power modes, etc. and are being breakpoint-stopped by the debugger.
The "IntDefaultHandler" is a function that is jumped to whenever an unhandled interrupt occurs. This could include the I2C interrupts, a timer interrupt, a watchdog timer interrupt, etc.
Check and see that you have interrupt handlers for all interrupts you are using in your project.