Thank you for your bug report.
After some trial and error I was finally able to reproduce this issue on Windows 10.
Below is the updated program executable that should resolve the problem, please let me know.
Download ffxp5.3.0.3929b.zip
Unzip the flashfxp.exe into your FlashFXP program folder replacing the existing flashfxp.exe
The problem occurred because the initial value of the item imageindex was greater than 1 which was not handled correctly on Windows 10