STM32F105 USB problem

I use a STM32F105. On the board there is a bootloader and I upload the firmware through USB (host, OTG_FS). Then I use the USB to extract a log file (.txt). The extraction of the log works only if the optimization of the code is 0. If I set optimization 1, 2 or 3 micro restarts while USB is inserted for the log extraction. I need to make it work at least with optimization 1. The strange thing is that if I upload the firmware without bootloader on the board, it works also with optimization 3.